Too late... around 100 million PC gamers already have Steam on their system and bought games on Steam (or Steam-codes at GreenManGaming, Humble-Store, indie bundles or key sellers). So why should they ditch their Steam-game-collection and migrate to the Windows/Xbox-Store? There is no Microsoft-advantage for this group.
Then there is the group of PC gamers who hate DRM and account binding or prefer retail games. If they missed out on Steam for that, they will probably also miss out on the Windows Store (DRM, account binding, no physical discs).
The only people Microsoft could win with the convenience of a pre-installed marketplace would be people new to PC gaming, but even most of them would be aware of Steam and its deals if they aren't cut off from the outside world.
