Correct me if I'm wrong, but doesn't Bruce Wayne 'retire' prior to the events of the Dark Knight Returns due to Jason Todd's death or something like that?
It's been a long while, but I figured isn't that enough precedent to do something like that in the film, especially considering how much Nolan was inspired by that particular graphic novel?
A lot of people conveniently forget about Dark Knight Returns, but that's a good point.