The kind of gamers that Valve courts are the sort of people who still build their own PCs, and who still interact with their computer through game controllers and keyboard/mouse. You aren't going to replace TF2 or Skyrim input with touch-based input. Notta gonna happen.

Just look at the games that Microsoft is currently peddling with their App Store. This stuff is not a competitor to Steam. It isn't even in the same market.

Yeah... you're focusing on the world of Candy Crush and <insert crappy in game purchase model game here> type games. Sure, huge market, also completely and utterly separated from the world of real gamers by a massive void of nothingness. There is of course overlap, but I'm not playing mobile games on my PC and I'm certainly not playing Arkham Origins or FFIV on some tiny tablet.

This isn't about "most people". This is about pc gamers, who still build their own boxes and are even borderline fanatical about their hardware. They aren't going to Best Buy and coming home with a touch UI by mistake. They are ordering parts from Newegg and building liquid cooling rigs in their basement.