I'm not sure what MS would be losing, if anything. But this thread is about how this is so great for Xbox owners. So....
It's funny you bring up the Vita. I'm eager to hear the argument that porting formerly-exclusive Vita games to the PS4 like Gravity Rush or Tearaway somehow helps the Vita or benefits the Vita. It doesn't. I'm not saying it hurts the Vita. Heck, it may simply be a symptom that the Vita isn't doing so hot. But the last thing I'd assume is that it means good things for Vita owners.
Most people buy consoles -- hardware in general, actually -- under the assumption that they'll get use out of it. If you think Xbox fans bought the system for Ryse and Forza 5 with no thought about Halo or Gears and other future exclusives, you're crazy.
We've already had a few of those and there are more coming, so those folks who bought in already got value out of their purchase and they'll continue getting value of it because those games are coming to Xbox One as well.