It would have ruined the movie.

You can't just put on a cape and cowl and be Batman. It took Bruce 7 years before he felt he was ready. Blake becoming Batman without any training would have been a gigantic stretch and just would have come off as cheap and corny. Nolan's better than that.

And Batman doesn't die. Even if it had been an Azrael type of thing, I don't think it would have worked or made sense.

And Bruce will never stop being Batman. Taking such a long break from being Batman did strike me as a little odd, but I just don't see him passing down the mantle to anyone else that soon, when he's still capable, and I still don't think he necessarily did in the end. he talked about it with Harvey, but he did have Rachel in mind. He was always searching for it, but TDKR shows he is never able to move on. And I can't see Bruce hanging with Selena and taking Robin under his wing without putting the mask back on. Bruce finding peace was never part of any Batman mythology.