Piracy: ꜱᴀɪʟ ᴛʜᴇ ʜɪɢʜ ꜱᴇᴀꜱ

I doubt any console is lasting 20 years nowadays.

I would recommend yuzu and roms if you want to future proof it, including the source code just in case.

I would lean towards either a DS, or switch emulation (using a device such as the Steam Deck, so games that require gyro input such as Super Mario 3D World - can still be played fine)

The DS is really affordable second-hand, and the usual third party SD card carts work just as well. People seem to really like the XL models, so may be worth considering one of those

As for the Switch, you'll likely be able to play your existing physical carts in 20 years time. It's still Nintendo's latest console though, so there's no telling what they may decide to do in future. I think if you have a jailbroken switch you could be in for a difficult time if newer carts use different hardware encryption keys that require a newer firmware...

With emulation though you get to actually own the games and play them on whatever device you want, at any time. The Mii maker and gyro setup does require following some guides to get going, but this stuff is well supported for the Deck (probably similar story for the alternative Windows handhelds but haven't checked). Yuzu's early access Android app already has gyro configured, using the sensors built into your phone. Lastly you can do multiplayer with other Yuzu players over the internet, completely independent of Nintendo's online functionality

People who own a Switch, let’s take this to extremes, do you feel like in 20 years from now you can still do the same things on your hardware as you can do now? (No multiplayer is fine)

No chance.

Not only it is unlikely that the hardware will last that long, the supplies for hardmodding it likely won't either, and in 20 years there won't be enough of a community interest to support hardmodding services unless some sort of master keys are leaked. And without hardmodding, the only Switches that you can install whatever you want on are very ld ones that were released with firmware 3.x or something, which are also less capable hardware and lower quality joycons.

Heck, if I had to bet on 5 years more instead of 20, I'd flat out sell my Switch and buy a Steam Deck 2.0 as soon as they release.

My switch died after about 4 years. A capacitor burned, tried to replace it, but still nothing happens... dont know if Ill get it to work again

1. It's pretty much needed only for online stuff, so single-player only is no problem. Also cloud saves.
2. Yes, multiple local accounts are possible.
3. Probably, since I no longer pay for online services, so nothing much will change for me.