I don’t think Proton is Valves endgame for Linux gaming.
It is necessary to kick off SteamOS, but in I’d guess they are betting on capturing market share with their devices and OS, so that developers will put in the time to do Linux ports.
Proton is always a catch-up game and can’t seem to be the end-all solution.
If Valve managed to acquire a good marker share, developers will care about having something that works in Valve's SteamOS, Microsoft will push developers to native ports if they tried to degrade Proton ports, so I think they'll prefer to keep their control over DirecX.
It is necessary to kick off SteamOS, but in I’d guess they are betting on capturing market share with their devices and OS, so that developers will put in the time to do Linux ports.
Proton is always a catch-up game and can’t seem to be the end-all solution.