I think the only way Microsoft would back PC gaming is if they had there hands dipped tightly inside one of the video card or CPU organisations. This in fact would make PC gaming more popular than ever, probably not better though

but as things are currently they appear to have no reason whatsoever to promote or influence PC gaming. This is where the technology is and this is where things advance so quickly, but this is not where the quick $$$ are.
The 'possible' death of PC gaming is going to effect so much more than just gamers. No office in the world really cares what video card's, soundcards or CPU's they are running unless they are in a specialised field where requirements are met so wham goes the market for these premier components.
I'd be hard pressed to find a PC gamer who'll even care about the new version of Windows if the above happens. Can you see Nvidia or AMD hard at work day and night on new display drivers for the sake of old games and artists/developers or studios, then showing off what there new cards can do in Direct X when there are no actual games to use them with.
So bearing this in mind I know there will always be PC games as long as there are PC's. Regarding empty shelves in stores. I'd not worry about this as the next generation of consoles are almost certain to be heavily involved in downloadable games and the shelves will slowly degrade for those too.
Having said that it's a damn shame Alan Wake is 360 only now, especially as it was once stated to take advantage of PC technology.