The games that don't run in general it boils down to terrible DRM schemes. If Valve moves the number of these that they appear poised to move... I suspect things like DRM that get in the way of Proton are going to be revisited by publishers wanting their games to run and be show pieces on Deck.Valve didn't fix the issues of playing Windows games with Proton because The Stanley Parable still won't run via proton.
Also your mistaken... The Stanley Parable runs well. Ironically its the badly done Linux port that has issues.
Steam has been doing a ton of heavy lifting themselves for a few years now. This is the point where they move a ton of hardware... and get publishers on board. Valve has set it up that to make it painless for them. Most of them won't have to do a damn thing.... however for a few years now Valve has been trying to get developers to create Proton configs themselves. This should crate the inflection point. Where it makes sense for developers to spend a few days to ensure their proton profiles are up to snuff.
I suspect a lot of people will install windows on these things... BUT most will not. SteamOS is back in a big way.