A Switch 2 with access to the entirety of the Switch's digital and physical library in addition to anything new on Switch 2 sounds like a recipe for one of the greatest consoles in all of gaming. Heck, factor in Wii U ports and it'll technically have access to three gens' worth of games.
You gotta think bigger than that!
Assuming Nintendo continues NSO now that they have finally established an actual online service, the Switch successor could potentially have
NES (NSO)
SNES (NSO)
GameBoy Color/Advance (rumored to join NSO this fall)
N64 (NSO or eShop)
GameCube (traditional BC via remasters)
Wii (traditional BC via remasters)
Wii U (traditional BC via remasters)
Switch (traditional BC)
Also worth taking into consideration that the Wii U was able to play DS games via the eShop, so there's not really any reason for Switch to not eventually get those too via NSO or eShop.
I have no idea what kind of technical issues there are with emulating 3DS, but if they can't emulate those games and instead port some of them, it's entirely possible that the next Switch would be able to play games from
every past Nintendo system. O: