I see it as the opposite. Batman is known to go on forever. That's the norm. So rebelling against that by giving him a happy ending is therefore the risky, new route. The safe choice would have been to give a boring ending where Batman/Bruce Wayne is looking over the city like we have gotten and will get for the next 100 Batman movies. You'll see, in 40 years from now you will realize that ending a Batman movie with him flying off a rooftop or standing on top a skyscraper is now the normal, safe, comfortable ending and extremely predictable one. It will happen over and over again, and you and i may love it every single time. But it's still safe. Actually every superhero movie tends to end in almost the same fashion. TDKR supplied something different. And it won't be repeated for Batman.
The other thing you guys are forgetting, is that this version was always about the symbol, and not necessarily about Bruce Wayne being the one and only Batman. Batman still continues at the end of Rises. It's still a Batman ending. The final shot is in the cave, suggesting the cowl lives on to fight an endless war. But some fanboys want their Bruce as Batman forever, but that's just brainwashing from the comics. Because they must carry through. It's about money over story, even if the writer can come up with some deep material in the process.
It probably didn't interest Nolan to end the 3rd part of his series practically the same way as Godfather 3 where Michael dies an old & bitter man. But this time with Bruce. Ive seen that before in one of our biggest trilogies we have. Or Nolan not ending it with a cop-out. Promoting the trilogy as the end yet giving people the same old superhero movie ending where Bruce is still batman, so WB can just re-cast and keep putting out sequels. How is that a real trilogy? How is that an ending to a story?
Never cater to the fans. Ever. Tell the story you want to tell and deal with the reactions later. The cockiest thing about this is that the fans truly think these movies were made for them first and foremost. They weren't. And thank goodness, because **** wouldn't get done. Things would be stale. Just like when a musician or pop star makes a record for their fanbase..it's compromised art.
I doubt Nolan cares if some fans hated the happy ending. Good for him. Good for Burton if he doesn't care what i think about Batman Returns. Thank God he did something different and told his story.