Think about the possibilities of merging universes. Fox can make Excalibur. Marvel can make New Warriors and Annihilators. Marvel can distribute War of Kings and Galactic Storm while Fox can make Secret Wars.
Also, Marvel are already working out a similar deal with Sony so Spider-Man can join the MCU which means we'll be able to get Thunderbolts now with Mach and Green Goblin means that Dark Reign may happen to say nothing of Spider-Man teaming up with Daredevil.
Or if Marvel did get the X-Men back, there would be no way that they could fit them in to the MCU while doing them justice since there aren't enough film slots to do justice to the various X-teams, nor would Wolverine and Deadpool get their own films.
Why couldn't Fox make Excalibur now?
Why couldn't Marvel make New Mutants now?
Why wouldn't Marvel make Secret Wars using the numerous Avenger and GotG characters? Yes, it would be great for Spiderman to be in it, and the original story had the X-Men and FF, but couldn't Marvel pull it off without them? Yes, Doom was a major player in that story, but they could change it up and still pull off the movie, without having to give up anything to Fox.
Spiderman teaming up with Daredevil? Why would Marvel do that? If anything they just need to make a better mouse trap as far as Daredevil is concerned, and maybe that is through a TV series.
Again, there is NO WAY that Fox will ever give up the X-Men rights. The $ changing hands for that to happen would have to be crazy! So it is what it is. Fox has the X-Men and have done ok things with them recently, though this trip through the decades is a bit weird and convoluted. And Sony has Spiderman and other than Secret Wars there's no huge story that I can remember where a deal would be worthwhile. Again, Sony is making big $ with Spiderman, so they will never give it up unless the $ changing hands is crazt. Maybe Disney will purchase Sony!!!!!
So I'm not really sure where your post was going. Financially there is no reason for Sony to give up Spiderman and no reason for Fox to give up either the FF or the X-Men. Any crossover movie that might be used to create a crossover movie would be very difficult financially and would just make more sense for Disney to do it themselves using their Avengers, Guardians and other characters. People just need to accept that it ain't happening!