They would have to pay all of the 3rd parties more money for their games to appear on other platforms I'm sure, it's not just as easy as trying to get Sony to allow it. When it comes to games like Elder Scrolls I also don't see any real value in GP over buying the game, those are games that people play for months leave and then come back to later, you'd be better off in the long run just buying it especially if GP goes up in price. They are good games to keep people signed up to GP but at the same time even if I were an xbox owner I'd probably just buy them as I have no interest in the rest of the GP library.