I disagree about the Joker. Killing him and letting him stay dead would seem about as nonsensical to me as trying to kill, say, Catwoman. The word 'iconic' is rapidly becomign wanky, but in this case it seems appropriate to me. Joker is one of the four best known characters of the Batman mythos, even to people who have never opened a comic in their life and don't know Two-Face or Batgirl.
On top of that, the rules of the genre mean that it's more convenient and often more effective for the writers to have only a handful of villains to work with - they're more immediately recognizable, and they mean something to the reader. It's a name that means something. When a plot involves Joker, we know to be scared the same way we know to be scared for the heroes when they're facing Shiva or Deathstroke. In a genre where the heroes always win, they're threats because they can win. That makes reading comics more fun, to me.