I agree that backwards compatibility on the Super Switch is definitely possible.I’ll just say this:
You have freelance developers who do this for fun and made an emulator on PC for the wider audience to use if they want
And you have two multi-billion dollar companies, one who made the original soc of the device and has documentation for it, and the other who documents literally everything that happens on their platforms.
I’ll leave for you to decide if this backwards compatibility is impossible/not happening.
I highly doubt Nintendo/Nvidia will go the route of full software emulation though. It's just too complicated to have perfect compatibility. I understand that some people make switch emulators on their free time, but it is far from perfect. If you want to emulate games from recent systems, prepare yourself to often (depends on the game) face issues, either graphical glitches, audio distorted/not working, crashes, etc.
If you look at the Dolphin emulator (which emulates GameCube and Wii), the official game compatibility list shows that over 60% of tested games have minor graphical/audio glitches (they can still be fully completed, though), and Dolphin has been in development for almost 2 decades. Emulating a Switch is significantly more difficult than GameCube and Wii.
And I think 2 other major problems here are expectation and cost: Most emulators are free and people should understand that if something doesn't work as expected, it's okay. They maybe wasted a bit of time, but that's pretty much it. On the other hand, if someone buys an expensive Super Switch, tries to play a Base Switch game, and finds some glitches and/or the game crashes from time to time, even though that same game was working perfectly on their cheaper Base Switch, they will probably complain. People have big expectations when they buy expensive hardware, especially from a multi-billion dollar company.
We can just look at the backlash Nintendo received when some emulated Nintendo 64 games on Nintendo Switch Online had graphical issues.
Testing would also become a major issue. Since we want perfect emulation, every single possible scenario in every single Base Switch game ever released or releasing in the future would need to be tested and confirmed to be fully functional. That same testing would also need to be done every time the emulator receives an update to avoid regressions.
Okay, I'm probably exaggerating here, but still.
Those are the same reasons I would be very surprised if Sony actually released a PS3 emulator for PS Plus, instead of the current streaming solution.