The story I heard a couple of years ago was that after "The Dark Knight," Bale asked his lawyers to look at his contract and see if there was any way he could legitimately avoid making a third Batman film, and they said, "Nope, you're stuck with a three-picture deal. If the third picture goes into production, you've got to be there!"
So once his contract has been fulfilled by three Batman films in a row, I do believe Bale will move on to other things, and someone else will need to be Batman in any subsequent film.
I've never understood this kind of thinking. They are playing the best character they'll ever play in their life. Seriously. Everything they do after may be good but it will be an afterward. Indiana Jones, James Bond, Doctor Who, once you've accepted these roles you will always be primarily associated with these characters. Why don't actors accept this at the outset? Otherwise maybe they aren't the right choice for the role perhaps?
I get that major characters like these are time consuming and an actor likes to play many major roles, and this is limited, but I don't think it is THAT limited. It came with the role when they took the money and popularity the role gave them. I get that Bale is a respected actor and was before Bats in the Machinist and playing a nasty piece of work in 'Shaft' but I don't get why you'd choose to be BATMAN and then go, hmm I miss being paid less to play marginal characters in movies less people will see or want to see.
However I wish Bale well, he is one of the few actors out there leading in films who is not part of the 'pretty boy' takeover, hence why I'll probably always be part of his audience. There is a lack of decent films and leading men at the moment anyway.