SteamOS is about Valve not paying Microsoft a license fee. It's not some egalitarian move by them but a cost saving. From a game dev's perspective it's additional cost and effort to port a game, or hope it runs under emulation with Proton.

I am surprised they haven't used it for cloud gaming yet since that's where the real cost savings kick in but I bet they're still saving on each Steam Deck they sell without Windows on it.