They pushed publishers to develop linux ports, they hired developers, they created a linux gaming distribution (steamos and the "failed" steamos console) and do their own research and development like ACO [0].
Proton in the end is "only" wine bundled with a good patchset and preinstalled debendencies, but even in the beginning you could get a good gaming experience out of the box that you only got through manual patching and tuning with wine.
[0] https://steamcommunity.com/app/221410/discussions/0/16409152...
I agree that steamos hasn't been a big success in the market, but I'm pretty sure Valve intended it as negotiating leverage - BATNA [1] - when Microsoft came out with Windows Store.
So long as Microsoft isn't declaring "No app stores except ours" or "30% cut of sales to us, no exceptions" steamos has been a success from Valve's point of view :)
[1] https://en.wikipedia.org/wiki/Best_alternative_to_a_negotiat...
Tbh I even got a wrapper script to try games with proton when they don't run well with wine just to see if it works any better and the wrapper is mostly setting environment variables and reading paths.
Both! They gained the advantage of 20 years of work on Wine before they started distributing it. Now they're continuing to push Wine forward by funding further development. Most work on Proton is sent to upstream Wine first, and is then merged back into Proton. All of this is visible in the public repositories.
This is exactly how open source should work :) Valve's Linux team are good open source citizens, you can rest easy.