If this film does bomb both financially and critically then the damage done to the Fantastic Four IP/brand would be such that Disney wouldn't really see the value in trying to 'buy back' the theatrical rights from Fox. And if Fox don't want to make a sequel (or yet another pseudo reboot) they could potentially just stick the F4 actors in a larger "Fox does Marvel" event film given how flexible their sequel options probably are.
I simply don't see any situation (in the short term at least) where this film's performance results in Fox and Marvel coming to some kind of arrangement with the Fantastic Four. Fox is a very difficult place to do business and, unlike Sony Pictures, F4 isn't all they have in their franchise cupboard.
Could an apathetic Fox let the clock run out on their rights to the F4 much like they did with Daredevil? Perhaps. But given how Disney are positioning Inhumans as a replacement in every way for the X-Men I don't see them looking to play nice with Fox anytime soon.