A lot of these rights contracts seem to operate under the agreement that as long as whomever the holds the rights to the film property keeps making movies that make money and don't damage the brand (in which case I think the creator or license distributor could sue for the rights back if they wanted) - then yep, they can hold onto the characters for as long as they want.
Remember, when Marvel signed all these deals with the likes of Sony and Fox, they were recovering from near-bankruptcy and gave the film rights out to all of their top-tier characters (Spider-man, X-men, Fantastic Four, etc) in what you could probably consider an act of desperation on their part. They didn't put any sort of "we get the rights back no matter what in ten years"-clause into the contracts because, well, why would Marvel's top brass circa-1997 have ever considered the idea that one day Marvel would open their own film studio? Of course they'd sell the film rights to Spider-man off, what were Marvel going to ever do with it except sit on it? No one at the time could have possibly predicted the super-hero film boom on the horizon.
I think that's something a lot of people forget; feel free to get annoyed or angry with Marvel that that they have all of these stupid contracts with other studios involving their biggest characters, but it was those deals that likely helped a great deal in keeping Marvel from declaring bankruptcy and going the way of Cross-gen comics.
