That's kinda what separates consoles from PC. If you have a console, you never have to worry if the game you have will be compatible. If it says Wii on the box, it'll work on your Wii. If it says Xbox One, it works on your Xbox One.
PC doesn't have that luxury because of so many variations. You can buy a game but then you have to worry about minimal specs and that's the kind of problem you'd be creating with a modular Switch. We've seen issues like this on a smaller scale with N64 and the Ram Pak (certain games improved while other games couldn't run without it) and various console versions and peripherals throughout the years (GBC and GB) but what you're suggesting is basically a Sega 32-X and it would just lead to confusion.