Disney wouldn't bother getting the rights to the Fox fanfare if they were releasing the movie themselves.

What I could see happening is Disney giving Fox theatrical distribution on the ST in exchange for getting full distribution on I-VI right now. Fox gets to make money they otherwise wouldn't have, Disney gets the existing movies, and we get our fanfare. Everyone wins! The situation for Disney would be no different than it was for Lucas on the prequels.

Of course, Fox will probably ask for some additional upfront payment before they completely let go of I-VI, but the two companies would be foolish to not at least consider this idea.