Take the year 2018 for example: right now the Marvel titles that are set to be released include Black Panther, The Avengers: Infinity War, Ant-Man & The Wasp, New Mutants, Deadpool 2, and X-Men: Dark Phoenix.
It would be impossible for any one studio to release all of these movies in one calendar year – which is to say that if all of the rights were to revert, we would potentially wind up seeing our favorite characters less often.
As cool as it would be to see the X-Men, Fantastic Four and Avengers team up on the big screen – as they have for decades in the comics – it should be noted that there would be a serious downside to having all of the movies made by just one studio.