Nintendo has been quite keen on backwards compatibility across quite a few devices, particularly those based around the same general architecture. If this rumored Switch 2 A) exists, and B) has physical capabilities the same or comparable to the Switch (using Joy Cons and controller compatibility, being able to function in docked and handheld modes, etc.) it's a pretty good bet that it will also be able to play Switch games.

We can hope, perhaps, that in its backwards compatibility mode it might even be able to get Tears of the Kingdom to run at a decent frame rate. The fanboys will hit the moon if so.

The Gameboy Advance could play Gameboy games. The DS could play Gameboy Advance games. The 3DS could play normal DS games. (And DSi games, but no one cared about those.) The Wii could play Gamecube games. The WiiU could play regular Wii games. Nintendo's track record on backwards compatibility, at least across a single system generation, has historically been quite good.

On the digital front, you could transfer your digital stuff from the Wii to the WiiU, and you could also (I believe the plug on this capability has since been pulled) transfer your stuff from a DSi to a 3DS. Again, if the architecture is the same I think transfer of your digital assets will also likely be possible.