ithilien827: People seem to forget that Steam has a huge lock-in-advantage, and you also have to factor in that Microsoft will never be allowed to do something that comes in the way of fair trade.
What is this "lock-in" advantage that you speak of? I don't think most PC gamers refused to buy e.g. Mass Effect 3, because it is not available on Steam. It is not that unthinkable that people use more than one client for digital purchases, and may migrate to a new one _over time_, keeping the old client around mainly just to access their older purchases (which does not bring money to that older service provider, only buying new games from their service does).
Apple is allowed to prevent people from buying and installing software from other sources besides their official app store in iOS, and MS can always use that as the argument that they should be allowed the same. That is "fair trade", level playing field for all the competitors.
ithilien827: You also have to think about the public outcry if the majority of pc gamers lose access to their games. If this happens, and the rights owners don't reticify the situation, piracy will become the norm. People will find it ok to pirate the games they already own, and when that ball is rolling it won't stop there.
Legally, the complainers have no leg to stand on. They just bought terminable licenses from a digital service. It is no different than how in OnLive you can lose access to many games you have "purchased" before, OnLive promises only two years of access after purchase, I think. It may not feel fair to the complainers, but that's only because they were ignorant, thinking that they have any kind of eternal "right" to the games they bought licenses for from a digital service.
And things can proceed step by step, it doesn't necessarily happen overnight. The digital publisher can slowly change their TOS, introduces new mode of operation that slowly replaces the old service etc.
And it may be the new rivals, e.g. Windows Store, will help the migration by offering you to get _some_ of your Steam/Origin/whatever games for free from their service.
As for piracy argument, I guess that is why we see more and more ISP blocking for pirate sites.