Earlier quoted context omitted.
There's a good chance that if that if Microsoft doesn't act soon enough, and a lot more devices running Steam OS are released, Proton might become the de-facto platform against which many new games are developed, and which engines target. At that point, there is nothing Microsoft can do.
Microsoft controls Windows and DirectX, Valve only gets to play until Windows landlord allows it. DR-DOS, OS/2 and EEE PC. Lets see if SteamOS makes the list as well, this is after all round two, Steam Machines didn't go that well.
DirectX has to stay reasonably close to Vulkan. And Vulkan is not an afterthought for graphics card manufacturers, quite unlike OpenGL of yore.
And Win32 (sans Vulkan/DX) is mostly feature-complete for gaming purposes. Manufacturers can just target the current state of Win32 for a decade more, if not even longer.