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.