You might not even need the Windows install at all for gaming. With all the work Valve has been putting into Proton, even games with funky anti-cheat should work on any Linux distro by the time the Steam Deck comes out.
I haven't kept a Windows machine for gaming since 2015. It was hard then, but now it's a breeze if the game is on Steam. If it's not, Lutris will probably handle it for you. Most single player and a lot of multiplayer games work without a hitch. I am, for instance, playing Tetris Effect connected and Dark Souls 3 online right now.
While there definitely are a lot of games available, a large fraction still are not. Having a Windows install is not a huge deal.