Bolded: I don't think this is true at all.

As for the DS vs. a more powerful GBA, the main point of my previous post was that Nintendo needs a different value proposition when they are up against competitors who leverage other business divisions to subsidize their console hardware, i.e. sell it a significant loss to bolster their market shares. The DS was indeed a better choice than a more powerful GBA would have been, but that doesn't change that Switch's successor will not be in a similar situation as the GBA's successor has been. Now Nintendo already has a different value proposition which is hugely successful, that's why they don't need a major innovation.

The GBA was just a better GB and sold at a rapid pace, that's why I don't believe that the bolded portion of your post is true. The crucial part about Switch's successor won't have to be hardware choices, but rather software choices, because it has never come down to hardware choices alone for Nintendo. Their weaker selling systems weren't solely held back by their respective hardware, but also sequels to their popular IPs that weren't exactly what the market wanted, or sometimes even the refusal to make sequels altogether.

If Switch's successor is merely a better Switch all around at a fair price ($399 or less) with a solid launch lineup and first year software release schedule, then Nintendo will be fine and won't have to worry about selling less than 100m units of hardware. But if Nintendo pursues something unappealing, such as VR which has been suggested a few times in this thread, then it's very likely that we'll see a repeat of the 3DS where Nintendo will be quickly forced to sell the hardware at a loss in order to maintain confidence among their third party partners.

Switch's major standout feature on the hardware side is that you can keep playing the same game you just had on your TV completely hassle-free on the go. This has proven to be an incredibly valuable feature for men in a relationship, because it increased the amount of their playtime tremendously without pissing off their girlfriend/wife. This human behavior is not going to change, hence why there will remain strong demand for it.

I would be happy with that, but I guess the many casuals who bought a Switch won't upgrade for those reasons alone.
If Nintendo only upgrades the hardware without a new feature, I'd see sales below 100M.

To me, just like bitcoin is the end of history for money, the Switch is the end of history for consoles (Nintendo Consoles). Switch has almost everything I want from a console. No significant innovation is necessary this time around.

Then you're not thinking big enough. Switch 2 will have a lot to improve upon. And Bitcoin is a sham.

This is also fine, Switch had amongst other things the pandemic to boost it.    

More than anything Games will be what gets the Switch 2 above 100m. If it wasn't for animal Crossing connecting and blowing up the way it did, maybe the Switch would have sold closer to the PS4. Either way, I don't think Nintendo can top the Switch sales wise just as sony can't top the PS2 yet there's made more revenue with recent consoles because the higher amount of games being sold + subs.

Switch 2 will probably sell less than Switch 1 but have way more support and game sales. 

I don't know why exactly you'd disagree with that. We saw about half their handheld business drop out from DS to 3DS. Granted the 3D feature wasn't enticing, but ultimately, it was a product priced similarly to the DS (eventually), with a generational leap in graphics, and a pretty similar, and I'd argue superior, software lineup. I don't think you could blame the drop solely on the 3D feature, I think a large part of it was that people just got bored and needed something to motivate them.

We can also look at the Vita. Hardware wise, the Vita was a more powerful PSP, and really impressive for its time. It was basically what people were suggesting, a more powerful version of the previous hardware. And it completely failed, despite Nintendo losing so much market share.

You could of course point to other factors as one always could, but at the end of the day we had the handheld market contract by about half. Because not ever person who plays games necessarily does so as a part of their lifestyle, and not everyone who does so necessarily needs to play them on the go. 

As for the GBA, as I mentioned, the hardware leap here is far greater than what will be possible with a Switch 2. Maybe not in terms of number crunching, but in terms of how it appears to the human eye. It was essentially a ten year jump from a system that was far from cutting edge. If the Switch 2 was going to have that kind of leap, I might feel differently. Also, the GBA didn't sell exceptionally quickly compared to the Gameboy. If you compare it to the Gameboy's early years, then yes, but if you compare it to the last few years of the Gameboy, things are differently. The GBA basically just continued the trend of the Game Boy line. This was mainly due to the influence of Pokemon combining with the GBA hardware; there was now a large market for handheld titles and hardware that could run more ambitious games. The Switch 2 wouldn't have the same benefits.

When we look at when Nintendo has increased its marketshare, it is with the DS, the Wii, and the Switch, systems which had some unique feature. Except for arguably the GBA mentioned above, they've never increased marketshare by simply adding power and QOL. As you and a few others keep pointing out that focusing the Switch 2 on a feature nobody wants would potentially tank the system, as with the Wii U and 3DS. And sure that's true, but I'm not suggesting that Nintendo should make a bad feature. If they make a good one, they can potentially keep the same level of success as the Switch, which people here don't seem to think will happen with just a straight upgrade, or maybe even surpass it.

As for the girlfriend argument, I just don't know about that. I'm sure that's part of the appeal, but according to research, most of the Switch fanbase is female. https://www.gamesindustry.biz/eedar-nintendo-switch-attracting-more-women-wider-age-ranges-over-time  I didn't really look into the methodology, so grain of salt. I'm sure some there are some men owning a Switch for that reason, but I'm skeptical that's the main cause of the Switch's success. 

Lighting and resolution was an oversimplification. But, it seems you agree with the main point there, that the graphical upgrade will be smaller than the GB to GBA, or really any other Nintendo system. You really just couldn't do F-Zero on NES, Mario 64 on SNES, Luigi's Mansion on N64, or Golden Sun on Gameboy. Do you think the Switch 2 will have any software that you couldn't do on Switch without making major gameplay compromises? I don't think so. 

I get there are some people who care about the kinds of upgrades that would be possible, but the best selling games on Switch are Mario Kart 8 Deluxe, Animal Crossing New Horizons, Smash Bros Ultimate, Breath of the Wild, Mario Odyssey, Pokemon Sword/Shield, Scarlet Violet, Super Mario Party, Tears of the Kingdom, NSMBU, Ring Fit Adventure, Pokemon Diamond/Pearl, Legends Arceus, Splatoon 2, Luigi's Mansion 3, 3D World, Splatoon 3, Mario Party Superstars, and Switch Sports. Are the people buying these games, almost all of which can and most of which did, run on Wii U level hardware really going to be excited by what an improved chip could do? I dunno.

There is kind of a no true scotsman fallacy going on. For any system that decreases sales despite more powerful hardware, we're pointing to competition, software, or some other hardware choice. But with the 3DS and the Wii U, we're blaming it on the hardware feature/gimmick, and not looking at alternate explanations. And... the top games on the N64 didn't stand out? Huh? Mario 64? Kart 64? Ocarina of Time? Starfox 64? Banjo Kazooie? Smash Bros? Goldeneye? Mario Party? I'd say the Gamecube also had a pretty great first party lineup. If anything, these systems were more hurt by weak third party support.

We can do the same for the 3DS and Wii U. Whether or not the Gamepad was appealing, the Wii U had a pretty terrible software lineup, especially early on. It launched with NSMBU and Nintendoland (underrated gem), and then basically had nothing till Pikmin 3. By the time it got titles like Mario Kart 8 and 3D World (which I'd argue was still pretty weak) it was really too late. With the 3DS, the thing was absurdly overpriced at launch, show by the fact that they were able to cut it so drastically. Again, this did not have the best launch lineup, with Pilotwings, Steel Diver, and Nintedndogs being the first party titles. Neither were going to do as well as their predecessors regardless, but marketing and software decisions didn't help.

The market is complicated, and it's hard to identify how much each factor hurt a system. What we can say objectively is that Nintendo has never experienced growth with a straight up hardware iteration (possibly with an asterisk for GBA). The times they have experienced the sharpest declines have been on systems that had some unique feature (3DS, Wii U). But, the times they've shown the most growth have also been when they've had unique hardware features (DS, Wii, Switch). 

I think what we can fairly say is that making a system with better graphics along tends not to increase Nintendo's marketshare. Adding a gimmick/novel hardware feature can either help them tremendously or hurt them.

So, Nintendo can play it safe and just release a souped up Switch, it will likely lead to a decrease in marketshare. If Nintendo can think of a good new selling point, they can likely maintain or increase their marketshare. If they come up with a bad gimmick, it can be disastrous. Seems like most people here, if they were in charge of Nintendo, would play it safe. If I were in charge, I'd go for growth.

Wii U/PS3+ tier to PS4+ tier graphics can still be a pretty huge upgrade, to say you wouldn't notice a difference between this and a current Switch game ...

Depends on how you define huge. Of course, there's a difference. Enough to make people go "wow I really need to buy that new system"? Some yes, but not enough to prevent a sizeable sales drop IMO. Especially when you look at the best selling titles on the Switch. None of them are graphical juggernauts. 

I would say Mario Kart 8 Deluxe and Breath of the Wild were graphical powerhouses for their time considering they are portable games you can take in your coat pocket. There weren't too many smartphone games and nothing on a Vita that would match the visual scope and scale of those games. 

3D was very detrimental to the 3DS, it pushed people away from buying one. At first Nintendo did eat the cost, but eventually they also released the system without 3D. That speaks volumes about how offputting 3D was. Plus 3D also cut the battery life in half, so even people who didn't want to use it at all were still negatively affected. The 3DS's retail software library certainly couldn't compete with the DS's. There's a wide range of software you aren't interested in, but for a lot of others it did matter. Such as games based on licenses, no matter how crappy they are, but the video game industry moved them to smartphones based on their belief that mobile was the logical successor to the dedicated gaming handheld - it turned out it wasn't, but additional damage was done. There are more examples for how the library suffered, but in general a strong decrease in the overall number of released software goes hand in hand with a decrease in breadth and depth of a library.

The Vita failed to have proper R2/L2/R3/L3 buttons, so it wasn't a complete basic upgrade over the PSP - come to think of it, Nintendo also released the monstrosity Circle Pad Pro for the 3DS, because they hadn't thought through things either. Aside from this hardware blunder, the Vita was also negatively affected by the video game industry's assumption that mobile is the future. As such the Vita's game library paled very much in comparison to the PSP, especially as far as American and European software is concerned. The PSP also had the benefit of releasing before smartphones, so it could sell based on its multimedia features too; the Vita got none of that anymore.

I don't expect the hardware leap to sell Switch 2, it will still be its games. Gameplay-wise, we've basically hit the end of the road with the Wii U by making open world games comfortably doable. Doesn't matter though, because "comfort gaming" is a big thing. Comfort gaming is defined as something that you know, something that assuredly is fun for you. Just like there are certain games that people always tend to go back to over time, there are certain game types that will be always welcome. So if Nintendo makes sure that odd hardware choices aren't getting in the way, they'll keep that audience.

If Switch 2 is to Switch in sales what the GBA was to the GBC years, then I don't know what the problem is. There's now a large market for console gaming that seamlessly transitions from the TV to portable. People know it, people like it. Third parties know it too, so I doubt that Switch 2 will have to go through the same extent of hesitation as the first Switch.

Other people here aren't the measuring stick for what makes Switch 2 successful or the level of the success it will have. The track record of the general consensus on an internet gaming forum regarding Nintendo hardware sales predictions is very, very bad. Nintendo's current situation with their handheld monopoly isn't one that has held true often in the past - the GBA is the sole exception - nor is the current status quo of being the only one to have a hybrid console a common one. The important main point here is that Switch 2 will have a unique feature by default as the only hybrid console on the market. Or in other words, you don't need secret sauce for continued success when your well-known sauce is already something that the market can't get enough of.

The girlfriend argument is just one angle to explain why switching play modes is such a good thing. It's not all there is to Switch's success. Of course it's also true that Switch appeals to females.