Re: Fear. Chaos. Pain. Vote for your favorite Nolan Batfilm!
I do wonder if there'll ever be another CBM to do a "hard" ending. Thing is, Nolan was in a unique position of power. WB knew they needed him to come back and were willing to give him a ton of creative freedom. They also have a great working relationship with him and don't want to burn bridges by mucking around with his Bat-universe. But for most of the other franchises out there, it's the studio calling the shots. Certainly with the Marvel films, save maybe Joss Whedon. And studios don't usually want to end highly profitable franchises, they want to keep 'em going for as long as possible.
I definitely wouldn't be surprised to see more third superhero films try to raise the stakes as much as possible though. That's how it should be anyway.