But no, the Microsoft Experience is inviolate, the holiest of holies, eternally immutable. No matter how much hatred it gets, it Must. Not. Be. Changed. And then Alienware ships a Windows 8 PC that boots to Steam instead of Metro. SteamOS's job is done. When no-one was looking, Steam took Microsoft and snapped it like a twig.
Or Microsoft found out they must cede the battle to avoid losing the war. That doesn't mean Valve should get complacent, once you make a threat like that it'd better stay credible. If they back down too far Microsoft might try for a blitzkrieg shoving the Microsoft Store down users' throat before Valve has time to rekindle the SteamOS project. At the same time they don't want Steam to go mainstream to avoid making it a real enemy to Windows.