for some reason I get the impression that Nolan's franchise would have continued if Heath Ledger hadn't died. Audiences would have demanded seeing Batman and Ledger's Joker face off again. Nolan himself even set it up with the line "you and I, I think we are destined to do this forever". I also think he might have also let his Batman franchise be the springboard for the shared universe, but Ledger's death is what made him want to "close off " the series. I don't know, it is just an impression I get