While crossovers would be nice, Kevin Feige has said that Marvel Studios only plans on doing one or two movies a year. With that schedule in place, buying back the other franchises would mean fewer movies in total. The alternative would be to risk weakening the oversight that has kept the quality of the self-produced films so high.
Separate film universes with fairly consistent quality, or the possibility of crossover "event" movies (AvX, anyone...? "Civil War: The Movie"...?) within a smaller overall number of films )or, alternatively, the same number of likely lower-quality "rushed-out"-feeling films)... I know which one would get my vote.