

Maybe Emulation is something you would look into? I actually prefer emulating the games myself, over playing the official Steam version (not at least because of the Shaders). Also there are lot of Mods (also known as Romhacks) for these games.
I imagine they must have given these are playable on PC, and doing them with a mouse would be just short of impossible.
The Steam Input system allows you to customize controls with lot of features available. So if they butcher this up, you have at least tools built into the Steam Deck to customize it. But it requires some time and effort, and that’s not always someone wants to put into to play a classic game.
That’s true. But there are legit ways to purchase Roms and extract them for use in your favorite emulator. In example the SNES Classic console from Nintendo, or the Genesis and Sega Games collection on Steam. If I were to buy Castlevania Advance, I would probably extract the Roms for use in RetroArch.