It's stupid to think the switch will suddenly stop selling. No it will continue to have the sales trajectory it is meant to have with or without next gen. It depends more on what Nintendo does more than anything else and I mean software and PR. The tone and trajectory of the console is set in the first year or 2 and that's what happened with Xbone and PS4 and WiiU and almost all other consoles. Nintendo would have to screw up pretty badly for switch not to sell like a behemoth. The switch has a shot at a 100mil consoles and ps5 and series x(stupid name) wil not have any major impact on that number, only Ninty will. They should stick with the switch formula for at least 2 more generations hell even more as the power GAO's between probable and home consoles will keep shrinking.

That's like saying no other shooter game should sell well because COD is so popular or other open world games shouldnt do well because GTA is so big.

Smash Bros and Mortal Kombat are nothing alike and do not steal sales from one another and there is literally nothing that supports your argument.

Again, you cant take sales from one retailer in one country over a very specific time frame to make conclusions about games sales.

I know you are new here but over the last few years there has been alot of discussion and evidence pointing out how amazon rankings are almost meaningless when it comes to determining software sales.

I mentioned this in a thread long ago, but the Switch and PS5/Series X (Really stupid name, BTW), are going to have little to no effect on each other.
By next year, the Switch will likely be selling for $170/$250 between the Lite and main models and a massive library of games backing it, while the PS5 and XBSX will be going for $400-500 with new libraries starting they need to build up. The vast majority of people that are going to buy these two systems at launch & its first year are going to be the die-hard, loyal PlayStation/Xbox fans who either already have a Switch or are never going to buy it anyways. Meanwhile, the casual, mainstream gaming audience will be more focused on the Switch because it'll be at two cheap and affordable price ranges with large and growing gaming libraries that more than justify the purchase.
The kind of gamer that will wait 3-4 years for a price cut/revision and a substantial library before buying a system IS NOT the kind of gamer that will jump on a new, expensive system as soon as it launches. The Switch's launch didn't stop the PS4 from having its peak year. By the time the PS5 and XBSX get to that point where they'll be affordable and with a great, expanding library games - the exact same spot the Switch is at now, by that point the Switch will be on its last legs and Switch 2, or whatever Nintendo's next system will be, will have launched or just about to launch. And then THAT system will be in the same exact spot the PS5 and XBSX will be when they come out next year.
The Switch will inevitably see decline in sales in 2021 and 2022. But that's going to be more because it'll be entering its 5th and 6th years on the market on March 3rd of those years and it'll start to show its age. Just like the PS4 and XBO did in 2018 and 2019.

Does that really matter? Say, the PS6 drops BluRay and only banks on SSDs. You could say then Sony has left the optical disc drive console market, which they started their gaming enterprise on. Or an easier example: with PSVR, you might say Sony is leaving the home console market to enter the VR console market.

But would it matter? In the end all that counts is, they have a system which runs games and you want to play said games. Nothing else matters.

What is so funny, that back in 2007 when I entered VGC, you could read the same argument about power which you write here in regards to Wii and DS. Over and over. How the Wii and DS cannot succeed, how they are not powerful enough, yada, yada, yada. It is really a tell-tale sign of a noob in regards to console sales, if you argue with power as it had any meaning for the success of a system at all.

Towards your bolded Vita argument: no, adding a dock to the Vita isn't the same thing. Because the Vita fails to have the gaming experiences that make the Switch successful. The Switch would have been the same as the Vita (and similarly successful), if it hadn't Breath of the Wild, Mario Odyssey, Splatoon 2, Mario Kart 8 in it's first year. But it did. So it is nothing like the Vita.

I don't think the power difference between portable and non-portable will really shrink. A console that can draw power without regards to battery life and heat dispersal will always be more powerful.

But there is another effect at work: diminishing returns. Back in the day the power difference allowed for signifacant different gaming experiences. Take 3D capabilities for example, this allowed for completely new gaming experiences. Nowadays the power differences might bring you sharper looking images, more realistic physics or literally more grass in a scene. But that all doesn't impact the gaming experience in such a meaningful way. It does impact, but not as much as the difference between 2D and 3D. And Nintendo really drove home that point by launching the Switch with a vast open world adventure.

If the gaming experience Breath of the Wild offers is possible on Switch, there is not much it fails to deliver. And over time the difference between experiences provided by more power will fell less and less meaningful.

I would have agreed with this if Sony software sold the best too. The PS brand isn't just hardware sales, but software sales too, and Sony games sales is nowhere near the best selling in the world

Based on what? 3ds started to selling well after the price drop and new exclusive Monster Hunter games for a while, Switch have nothing of those but still sell very well and faster than 3ds in the same time frame

This will actually most likely change, probably not in 9th gen, since consoles will not be powerful enough, but it's fair to expect some smaller titles (which actually already happens to a degree).

What I'm referring to is switch (no pun intended) from polygon based worlds to fully volumetric ones (be it voxel octrees or something similar) - this is major paradigm shift, but (according to Tim Sweeney form Epic in an interview from some years ago on Gamasutra) requires some 25+ TFLOPS.

I wouldn't say Sony and MS are reliant on 3rd party, but rather that they are automatically covered on that front. When you buy a PS or an XB, you can be sure that you will always get most releases. That's actually a component of the brand loyalty with these machines, you know you won't miss out on the 3rd party stuff. On top of that, you get exclusives and franchises that will cement loyalty even further.