This will not decrease Xbox 360 sells.  Remember not EVERYONE has a computer capable of running Xbox360 games out of the box and most people rather play their games sitting on their sofa with their 50"+ Inch TV not in front of a computer.

This will in fact increase sales on xbox games (for the ones that don't own the console) which will be a good thing for Microsoft IF they keep it under control from piracy.  And like everything online.. It will not last too long before you can just download the game and play it no problem. After piracy unfolds it could backfire BUT like I said above.. NOT everyone is into the torrent, piracy.  Even with the huge amount of piracy artist still make money, movies still sell.  

I think it will be good :) but who knows. Time will tell.