It's not about dying instantly. Two Face died instantly. It's that Bane's death felt insignificant. His death didn't any more implications on the narrative or themes. It immediately cheapened the character as a villain. It cheapened the film.
It would have been far more powerful to have both Talia and Bane survive, broken themselves, knowing they had failed in their mission because they were unable to break Bruce Wayne. The final moments with them could have been used to crystallize Batman's ideas about killing, revenge, justice, etc. Instead we got cheap action movie deaths with none of the conceptual depth to their final moments that Ra's Al Ghul and Two-Face received. I think the final handling of Talia and Bane was my biggest disappointment with TDKR.
^ These two posts summarize it the best. It honestly just feels like squandered potential.
Begins made us a compelling promise for the series, that we would be playing with big villains that have big ideas, and the climax of these confrontations would
mean something philosophically. Leave a mark on your mind. Batman will have accomplished more than just 'ok, I beat the bad guys' for a change.
The Dark Knight took that promise and ran it even further down the field, gave us something philosophically
bigger with the climax. And not just with the Joker, but also Harvey.
Rises had the potential to keep doing that, but it simply dropped the idea. I cannot fathom why.
The original approach was why I had utter faith in Nolan. Even if I could nitpick little details about the first two movies that I would have done a bit differently, it was no biggie, because I thought I
knew he would still pull off something that would blow me away overall in this regard.
But apparently the old promise got lost somewhere in the process of making Rises. Bane deserved more, especially because they had begun crafting such a unique, impressive new take on him. What a waste of potential gold...
I'm becoming more convinced that this ending needed to be a two-parter to get more across.