I actually think the 'keeping the rights' idea is a little overstated. I think that if their primary motivation was having the rights long-term, they'd be investing more and creating a franchise. They might even be willing to take a loss on this film with the idea that they could set up for films that will be more profitable in the future.
I see this as more of an end-game. I think they have numbers somewhere that say the FF name, at this moment in time, is worth $XXX million at the box office so that if they keep the cost low enough, they can make a lousy but profitable film. Of course that will kill the value of the name, but I think they're playing the short-game and don't care what damage they do to the name long-term.
I would use an anology of a lake filled with bass. If you want to keep the bass population thriving, you can take a few out this year, a few out next year, etc. etc. etc. and always have fish. If you take all of them out this year, you'll have nothing left for the future.
I think Fox intends to take all of the bass out before returning the empty lake to Marvel.