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.