They wouldn't get exclusive access to those games immediately.
Licensing for multiplatform release would continue, at the very least, until whenever both expire (FIFA is 2022), and considering how expensive the FIFA license most likely is, I can't imagine MS is going to be willing to pay even more then whatever EA is paying.
In a situation like this, FIFA has the power advantage, not MS.
I also don't think that any sports entity is going to want exclusive games. They want mass appeal.
Even if that was the case, why the fuck would anyone pay $70 for one FIFA game on Playstation when you could get it as part of Game Pass for "free".
probably because a large demographic for games like Fifa only play a limited set of games, so gp could be useless.