Is that really a fair comparison? Switch Lite is almost surely cheaper to make in 2024 than Wii U was in 2016-January 2017. Switch Lite is one tablet with outdated specs. Wii U was already outdated of course for most of its life but there is the Game Pad and console itself to worry about. 

Now Switch and likely Switch OLED are probably as pricy to make right now as Wii U was around the end of its life, but even that's not a given. 

If Nintendo add backward compatibility feature then it will be a good point for nextgen. It will boost nextgen and slightly put pastgen in the shadow. Although I'm not sure that 100% of Switch's games are gonna be full BC (4000 games according to Wikipedia), I still believe that 70%+ are playable on nextgen

I'm not sure I follow.

What does the cost of Wii U have to do with the longevity of the Switch?

I'm fairly certain there will be full backwards compatibility - however, that's not what I was thinking about.

It would be logical for Nintendo to release all their games made for Switch 2 on Switch 1 as well in the first few years of Switch 2's lifecycle, given the huge Switch install base. Sony did this with PS5/PS4. Then, if Switch, after Switch 2 is launched, has serious price cut (to let's say $200 top), I can see it selling and living for few more years.
And then there's scenario where Nintendo stops supporting Switch 1 with games and lets it die prematurely, so they can push consumers into Switch 2 - but I really doubt they will do that.

It'll mainly come down to when the next Xbox releases. There has been talk of 2026 but until I see something concrete I don't expect it before 2027 and if it releases then it could be quite close where as if it's 2028 then the Switch should get discontinued first.

If the Switch is discontinued later it will also be interesting to know if Xbox van actually best the Switch in a single year. I expect the Switch to outsell Xbox series in 2025 and if the rumoured successor launches in 2026 we could as well add 2026.

However since I think the 2026 is unlikely at best I will vote for Xbox Series and estimate it will outsell Switch in 2026.

I dunno if they can pull of not being backwards compatible again without a load of critique. On the other hand ported Wii U games die make Nintendo a huge amount of money. The best selling Switch game released on Wii U first.

The main concern for Switch 1 is if it can run Switch 2 games properly its already struggling with Nintendo's own output of ambitious games. So they might push Switch 2 a bit harder than we think.

Well, I don't think it matters anymore what it costs to produce. It's more a business decision. They have all figured out that consumers will continue to pay launch prices for the duration of the cycle, so why lower it? That's the basis for my prediction, but we'll see.

If Switch hardware stays the same price and Switch 2 starts at $400 (I think $300 and $350 are off the table for Switch 2), Switch Lite sales as a percentage of models sold are going to skyrocket. While Switch Lite hasn't taken off as much as I expected it to, it's going to be a lot cheaper than Switch 2. That makes getting a legacy platform easier instead of stopping short $50-$100 of the new platform. 

Still, we'll have to see. If Nintendo wants to keep Switch in production for a while after Switch 2, they'll probably have to use price cuts or a bundle. 

So the consensus so far seems to be that folks expect the Switch to outlast the Series, despite being almost four years older.

Not that this would be entirely unprecedented, mind you; the SNES outlasted the Saturn and Dreamcast, the PS1 outlasted the Dreamcast and OG Xbox, NES outlasted first party production of the Megadrive/Genesis.