The thing is, steam's market dominance is one of user choice rather than anticompetitive strategies or lack of alternatives. Steam doesn't do exclusives, they don't charge you for external sales, they don't even prevent you from selling steam keys outside the platform, or users from launching non steam games in the client. The only real restriction is that access to steam services requires a license in the active steam account. Even valve-produced devices like the steam deck can install from other stores.

Sure, dominance is bad in an abstract theoretical way and it'd be nice if Gog, itch.io, etc were more competitive, but Steam is dominant because consumers actively choose it.