Cos 3DS stopped selling when the PS4/Xbone came out and DS when PS3/360 came out, right?

Yes, it is true that Switch had the best lineup ever for a console in year 1. Is that even in doubt?

What history shows us is that it's difficult to support two consoles at the same time without having any glaring gaps of multimillion-selling software. But now Nintendo doesn't have to go back and forth between two consoles anymore. There is only Switch, so that makes it a lot more feasible that Nintendo alone can sustain momentum.

I concede that having third party support is beneficial, but here it is important to focus on third party games that actually matter. You make the big mistake to assume that games that are big on PS and Xbox will be similarly important for Nintendo, but that's simply not the case based on historical sales data and the already available sales data for Switch. The reason why there is such a gap in importance is that third parties specifically tailor their software to gamers who avoid Nintendo, so on the flipside you get the result that people who buy Nintendo consoles aren't showing much interest in those third party games because they weren't made for them. An obvious example of this is that an M-rating is perceived as something valuable by PS and Xbox gamers while Nintendo gamers shrug their shoulders and tell you that a game is going to be good or bad regardless of its rating; in other words, glorification of violence is little to non-existent in the minds of Nintendo gamers. Most of the M-rated games don't offer much beyond violence, that's why they fail to gain traction on Nintendo consoles. The Resident Evil IP is one of the few M-rated series that manages to provide actual substance in its gameplay, that's why it is doing notably better than other M-rated games.

Additionally, video game history shows us that during the times Nintendo pursued the third party support you ask for, Nintendo's sales levels were at their worst. Both the GameCube and Wii U flopped hard and that's no coincidence when you take the above paragraph into account. Nintendo put more emphasis on games that their customers don't care much for, so customers turned their backs on Nintendo. Should Nintendo pursue that path again, they'll inevitably rush out a Switch successor that will predictably fail, because the vast majority of Switch owners will not be interested in moving on.

Remove all first party games on any console and see what happens. A console needs 1st party as well as third party to be a success. A bigger 1st party can make up for a smaller third party however, but's good to have a steady flow of both.

Anyway I don't see why the Switch isn't as powerful as any potential PS5 or Xbox Two means it won't get third party support. The Switch isn't as powerful as the current PS4 and Xbox One and it's getting the best third party support for a Nintendo console in years. If the Switch sells well, what reason is there for devs not to make titles built from the ground of or at least in mind of Switch's hardware?

For the Switch to stay competitive as a handheld it's fine for the next 8 years. I do think they'll need an updated system to truly keep the home console hybrid point going by 2025 however. I still think it would sell of course, Nintendo's handhelds do amazing. I just think the home console side would be very hindered compared to the handheld side by then. 

Can't see this happening. Sales will most likely drop after PS5/Xbox release. At that point Switch won't be up to date anymore so that Nintendo will need to release a new console.

These 'Switch will stopping selling when PS5/Nextbox come out' claims also seem to forget:

- NES kept selling after the Megadrive released

- SNES kept selling after the PS1 and Saturn released

- PS1 kept selling after the Dreamcast released

- PS2 kept selling after the Xbox 360 released

Basically, it's not at all unusual for successful hardware to continue selling after far more powerful hardware releases.

The Xbox 360 and PS3 games that came out in the first few years were better than what was on the previous systems - in some cases a lot better, but a majority of the big Triple A games still looked of 6th gen quality or didn't look that different and have aged terribly. By the time the PS5 comes out, Switch will not only have games that look significantly worse than a PS5 game already at it's launch, but nearly all of it's games will look worse than the PS4's games.

It's not really equivalent.

Switch will still not be as far behind as DS was behind PS3/360 or 3DS behind PS4/Xbone.