There are fitgirl repacks for most switch games. Emulators and all bundled.

I would be reasonably confident in offline games running in 20 years if you bought the cartridges, if you bought the estore versions I would be significantly less confident.

I would be shocked if the newer versions don't have a software hack way before that.

The fact that the first version was easy to hack made later versions lower priority, but at some point for the sake of preservation or to have the OLED, the new ones will catch up.

I've owned my switch since 2017 and Ive never used Nintendo's online services, I think they're actually DNS blocked or if I forgot to DNS block them then my console might be banned but it makes no difference to me, I get an error it can't connect to Nintendo when I start some games but other than having to click past that it's smooth sailing.

You can still have multiple users/profiles/saves without needing to link Nintendo accounts at all.

I think most of what I do with it now I could still do in 20 years although if I'm being totally honest one thing I use a lot is moonlight to remote stream games off my desktop and Im sure you could use it with current Gen PCs to stream but I'm guessing the between wifi and video codec standerds changing over time i dont think moonligbt will still work in 2044...but thats probably a bit outside the scope of your question.

An easier way to put it, the switch is currently probably the best modern console for piracy and that should tell you a lot about how little it depends on any kind of (not already cracked) authentication

I am trying to choose between buying a Nintendo Switch or a Nintendo DS.

Last time I checked a Nintendo DS is nowhere near as expensive as a Nintendo Switch, you could probably get both unless you only have $300 in your bank account.

would you remove the battery during those 20 years?

Get a Switch, get it right now, also get this tool. The answer currently is yes, as long as the hardware still works & the battery doesn't explode, and a switch combined with the MIG tool will give you a robust local library of games. Get your rig + MIG & never connect that switch to the internet ever again.

Nintendo is notoriously litigious & overly protective of all things Nintendo. The MIG Switch cartridge looks AMAZING but obviously Nintendo fucking hates it & they will probably try to issue a patch that nerfs it and/or kills your Switch in the future.

If you don't count multiplayer and buy only physical copies of your game, you will be able to play for as long the hardware will function. You can also hack your switch and keep a copy of every game you may need somewhere in an hard-drive too.

Assuming it is kept in working order, you'd be able to do everything you can do with it now, with the assumption the online services stay up. If they go down, you would lose your non-physical games and the ability to play online.

Your progression and everything is saved on the unit itself, and the hardware used likely will last for a long, long time if kept properly.

Almost nothing other than games you've bought on the store and online functionality in games is tied to your Switch account. And in 20 years time, who knows if that service will still be online.

i still use my super nintendo to play original secret of mana and link to the past, so i don't see why not.