>Valve certainly doesn't seem to give a shit

Valve is nonexistent in the modern gamedev community, Source 2 is used by approximately noone, and overall releasing one game every decade doesn't exactly make Valve a prominent voice in the gamedev community.

However, yes, companies lock their engines at whatever it was when they started their project, and any upgrades comes from internal engineering. Decade long undertakings like the "recent" FF7 remakes are still on UE4 and will stick with that for the third game, because that's what they started with, and at this point the rendering pipeline and workflow is theirs