Nolan's earliest interviews regarding a third film were regarding the topic of how you top the Dark Knight and that he wasn't ready to start work on a sequel...this is part of the reason why he's waited before starting work on a story. He's always said that he would do one more if they got a story they could all get behind and worked. So they must've found something they all agree is a good enough story to film or else I think he would have walked away from the franchise. The idea of him ending the trilogy with an actual conclusion intrigues me. You know Warner Bros will just reboot the franchise with a new creative team anyway.