As yourselves this. In fifty years of Marvel Comics, how many times has the FF and the X-Men crossed over without any anyone else in the core Marvel Universe (Avengers)? Now in that same period How many times have the Avengers (Characters) teamed up with the FF?? After you answer those questions you'll the realize that what Fox is attempting to do MAKES NO SENSE!! and there in lies my core complaint.
Well as I'm sure you're implying when Marvel has done really big stories - such as the Infinity Wars - they've brought in as many major characters as possible.
But if we're really realistic, the logical problem is with the many Marvel stories that don't involve more characters. The original Galactus story is a good example. Here we had Galactus setting up a device in the middle of Manhatten and the FF were the only ones who gave a damn. Where were Iron Man, Daredevil, Spider-Man, Captain America etc. etc. etc. when that was going on?
For the simplicity of story telling, tradition dictates that characters fight their own battles 99% of the time when the earth is in peril. That doesn't really make sense, but it's comic book reality.
In the case of X-Men and FF inhabiting the same univers and (presumably) being that universe's only superheros, it would make complete sense that they would team up if/when Galactus threatens the world. But while it might make logical, real-world sense, I do have a fear that so many characters could not be properly handled.
If it were completely up to me, I would probably keep the characters in different universes just so I wouldn't have to deal with the logistical nightmare of having 20 characters all battling side-by-side.
But I would also presume that if Marvel owned these characters, they would put them all in the same Universe that would lead to the same logistical problems that we have with the comic universe.
I don't know what exactly Fox plans, but I don't see any cross-overs as an inherently bad thing or inherently good thing. As with most things related to these films, it all comes down to how well it's handled.