1. X-Men (event movies only)
I don't see why this should be the case. Not every film that will feature the core X-Men team should be an event movie that will feature a cross-over from a spin-off. It would be very repetitive to the audience, event film+cross-over with another cast.
Fox could still do movies with the core X-Men team with a lower-scale like X2/X1/The Wolverine and still maintain box-office success.
"This is our chance to make a difference" - Sue Storm - The Invisible Woman
"He's stronger than any of us but he's not stronger than all of us" - Reed Richards - Mr. Fantastic
"We could these powers to help people" - Johnny Storm - The Human Torch
"You can't fix this, nobody can" - Ben Grimm - The Thing