Gamers are (I think) attracted to Steam because of it's convenience and price. Many gamers will pass up games on alternative dd platforms because they can't get the game to register on steam.
If Microsoft provides competitive pricing, fast downloads and good key management (easy key activation, no PC activation limits, DRM-free options, good online/offline story) I could see people switching away from Steam.
Amazon is actually trying quite hard in this area - their download application could use work, but they have pricing and good DRM policies in place. Plus, their community outreach is mind boggling (especially considering the other DD outlets do none at all).
