About 120-130, maybe 115. It'll be like a Game Boy/Game Boy Color to Game Boy Advance decline.
I don't think we'll see DS to 3DS or PS2 to PS3 declines by percentage. And Wii to Wii U is impossible.

It's basically guaranteed to be a success so a Wii U disaster is of the table. For how it'll do overall worse than the Switch is my expectation due to just how much that platform had going for it for most of its life. If the covid pandemic didn't happen the Switch 2 outselling it would be a lot more likely but I do think over 130m is more likely to happen than not so if it declines I don't expect it to matter that much. 

I'll add context to this part. For the SNES vs. NES the former declined cause of it having proper competition and the GB lasted over a decade before a successor was released while the GBA only lasted 3.5 years before that happened so if the GBA had a longer life it could've easily reached GB levels. And the other two declined so much due to Nintendo screwing up though both would've still declined regardless even if they were more appealing.

Between those four the Switch to Switch 2 looks like it could potentially be a combination of NES to SNES and GB to GBA since it'll likely have greater competition than its predecessor and probably won't have a massive phenomenon boosting sales.

I feel like it's way too early to give an exact answer; we still don't know its price or software which will be major factors, or how long it's life will be.

Just going off the hybrid form factor and expecting the usual Nintendo franchises and third party support equal to the current Switch, I'd say 120 million is the floor, but if it has some unexpected breakout hits, better-than-Switch third party support, an affordable price point, and lasts as long as the current Switch, I'd say 150m plus is on the cards.

The only problem here making those kind of predictions is the lack of insight into the actual future the next console will launch in.
Considering the clowns in the US office thinking of the tarrifs bit could definitely impact a part of the US Switch 2 sales long term.

Aside from that, I'm really expecting something similar to the SNES in terms of proportions (like most would for what is an iterative device).

Although, launch aligned it looks like it could get the early lead on its older brother going by rumored forecast.

Nonetheless, we're never under the looks of a pleasant surprise or a deceiving one.

Right now? I predict it will be a 2 to 173 year generation with zero to dozens of killer apps that will drive between 1 million and 37 billion sales.
But I'll let you know when I have more information to narrow it down a little.

I think it will do 3DS numbers at most. 3DS was a much bigger success than people give it credit for, so even saying that is very generous towards switch 2. The Switch 1 had no competition, but the Switch 2 has so much that it's a really hard sell. 3DS was discounted to 180$ shortly after it's launch due to poor sales, so how is a minimum 399$ system going to out sell a 180$ one?

I think it will do 3DS numbers at most. 3DS was a much bigger success than people give it credit for, so even saying that is very generous towards switch 2. The Switch 1 had no competition, but the Switch 2 has so much that it's a really hard sell. 3DS was discounted to 180$ shortly after it's launch due to poor sales, so how is a minimum 399$ system going to out sell a 180$ one?

I completely agree - the PS1 and other competitors significantly hampered the SNES sales. I'm sure you're right that GBA would have sold way more with a longer lifecycle. We don't know much about what Nintendo is planning, which is why there are so many factors at play here. The Wii U also had a super short life due to poor sales. I don't think that will happen with Switch 2, but time will tell!

3DS was a big success in Japan and more modest success elsewhere. I agree with price being a factor, but it's not likely the main factor. With Switch 1, the same logic would have said, "how is a $299 system going to outsell a $180 system?". We all know how that turned out!

Two big reasons the 3DS had a poor start is a weak launch lineup and its unique selling point being unappealing. Olivernintentoal already pointed out why the price argument is absurd so what big negative aspects will the Switch 2 have to make it underperform ever more than the 3DS did? As long as something as big as Mario Kart or the next 3D Mario is there at launch it's gonna sell well from the get go.

I think they're planning to basically repeat the Switch by that I mean have a huge first year which will give them enough momentum to have a guaranteed big success. Mario Kart 9, the next 3D Mario and the 2nd Mario film all releasing in a year span would do wonders for it alone so combine that with various other games and much better third party support from the get go and there's massive potential.