There is no guarantee that Nintendo will make the Switch replacement backward compatible with the current Switch, don't forget Nintendo do their own thing.
It could also be possible that Nintendo could ditch the cartridge for its next gen replacement making games digital only, I've got a good few games that are digital only on the Switch as it's more convenient than swapping carts all of the time.
If Nintendo switched to X86 then 3rd party devs would have a much more easier time converting games for Nintendo's next gen console, rather than having to rebuild a game to run on ARM, the Switch was basically chucked together using the Tegra X1 which was outdated back in 2017, because Nintendo didn't think it would sell that many Switches, after the poor sales of the Wii U.
Nintendo’s next gen console has to think 3rd party and as good as ARM is, "I currently have a M1 Macbook Pro and love it" but both the PS5 & Xbox Series S/X are built on X86 hardware, and as the Valve Deck has shown you can now get something as powerful as a PS4 in a handheld on x86 Architecture, In next 2 to 3 years it wouldn't surprise me that these kind of APU's are hitting close to 3 Teraflops while being very power efficient.
Also if they where to stick with Nvidia which they probably will, then it will have to be a custom APU built from the ground up just for Nintendo's next gen console.
The current Nvidia APU's aren’t designed for gaming more for the car market, and this itself will cost Nintendo a lot more as Nvidia don't do things on the cheap, than say switching to AMD which will more likely have a decent of the shelf APU, that Nintendo could use that could run at 4K or even 8K upscaled & use AMD's equivalent to DLSS which will improve over time, & could be just as good DLSS in the next 2 to 3 years.
But at the end of the day whichever way Nintendo goes I'll still buy the next gen console on day 1.