As others have alluded to, I think the situation is different exactly because there is at least a real potential for competition. Epic seems to take a good swing at this. Sure, a huge market share _favors_ Valve in this case, but that's nothing compared to the technical infeasibility of the same approach say on iOS.

The PC remains an open platform, even MacOS to a degree (albeit apparently less so as of late...) and Valve have aggressively campaigned for it to stay that way. Competition there is possible.

I remember the debates from ~10 years ago around Microsoft's moves towards a more AppStore-like experience on Windows, where Valve really led the charge against it. The move towards SteamOS and Steam Machines seemed to be part of their reaction at the time. And it seems to pay off big-time now, not just for them but also for users, Linux users especially of course. :)