gog.com also distributes games and explicitly does not use DRM. They sell many older titles that run in DOSBox, those should all work on Linux with new install packages. If a larger commercial Linux game market develops then game makers can choose which storefronts their games are available on, I don't think Steam generally requires exclusivity.
Also the usage of DRM in Steam is pretty benign as far as DRM goes, games are tied to your account and you can install them as many times on as many machines as you want as long as you log in. You can log in even when offline and you can make offline backups of your purchases. Unless your goal is to redistribute software and artwork that you don't have a valid redistribution license for it doesn't really get in your way. I'm not sure what the valid legal use cases would be that their use of DRM would prevent.