What fmalover said.
Speaking as someone who knows next to nothing about Marvel comics other than what I’ve seen over the years in other media (the early 90’s X-men cartoon was a Saturday morning staple for me as a kid) and picked up from friends of mine who actually do know the source material quite well, to me there’s a big element of “who cares?” when it comes to whether or not x-men and F4 ever cross over with the MCU. I’ve even heard it argued by actual fans that the “mutantverse” should basically be its own thing anyway.
Obviously it’s a little different with F4, whose absence in the MCU has been sorely felt by the hardcore fans as far as I can tell. MovieBob has a pretty good video on youtube about how he’d handle their introduction into the MCU. His solution? Make their origin story a time-travel story that flings them from the 1960’s into their future / our present.
Anyway, stuff like that could end up happening anyway without one company needing to buy out the other. The rights to Daredevil eventually went back to Marvel from Fox, didn’t it?