Now, in my opinion, this is a really stupid idea. Let's think about this for a second. If a die-hard X-men fan, who loves the comic books, can no longer get X-men comics then where are they going to go? They're going to go to the one place that has X-men, Fox. So if Marvel basically starts boycotting X-men it'll actually push people towards Fox. Sure there will be the fans that are sticklers and won't watch the movies Fox makes but many will just want some kind of X-men.
Honestly I think Marvel shouldn't have given the X-men rights to Fox in the first place because they've done a fairly awful job with the movies so far. They've basically made the entire X-men movie franchise about Wolverine. Don't get me wrong, I love Wolverine, but it's the X-men franchise, not the Wolverine franchise. I'm pretty sure Fox would be able to still sell the movies if they chose a different main character. Or even, I don't know, made it about the X-men as a team rather than an individual character. There's a novel thought. Make a movie titled after a team of mutants about more than one character. Who would of thought that was a good idea? The X-men have always been successful because fans were able to relate to them. The X-men were originally created to parallel prejudice and the idea that sometimes people go through things beyond their control and they're not always liked. It showed a family who could come together to make it through any obstacle. It showed the bonds of family even when they hated each other sometimes and fought over the most insignificant things. It showed family didn't have to be blood and that is why I love the X-men.