Unless Fox is willing to wait on releasing another primary X-Men movie until 2019 or later, there’s no way he could return to the world of mutants in a directorial role.
Even if he doesn’t helm any more of these movies, his expertise would still be valued as a producer or even as a story-crafter.
Plus, he has 20,000 Leagues Under the Sea and the Freddie Mercury biopic Bohemian Rhapsody to fill his schedule for the next couple years.
From a behind-the-scenes perspective, these rumored changes make some sense.