But it's not like you need SteamOS to take advantage of Proton. You can just make use of all the things they've added to it in any Linux distro. I know because I have SteamOS in a VM and have Proton working in it just fine playing stuff for experimentation.
As for all the other things they've added to the OS. Well, they're specific to the Steam Deck and hardly "portable" to a regular PC build. They can't release all of that without first trying to make it non-tailor made, no? Or at least, that's what I'd say. Happy to be wrong there, though.