I agree with pretty much everything you said, cap, flash and green lantern have all had successful replacements. BUT, we're talking about Batman here. When it comes to the three biggest superheroes in the industry: Batman, Superman and Spider-man, i think replacing them is much much harder than replacing the Flash or even Captain America. I could guarantee that if Superman was replaced with anyone other than clark kent, there would be an immense outrage from the fans, why?
So? There's outrage over every significant event in every comic. Outrage fades. Personally, I would have loved to see Kon-El become Superman some day. Too bad they killed him. The reason you can't replace Superman now is because 1) there's nobody to replace him (most of the time), and 2) replacing him probably won't make sense in the context of the story; Superman isn't going to get old and retire, and if even if he dies, nobody is going to say "I should be Superman now." The only other living Kryptonian isn't a man. With Kon-El, there was a rare situation where replacing Clark would have been possible and sensible--Kon-El was kryptonian (mostly), and Kon-El was genetically related and emotionally family to Clark. Moreover, being a product of Clark's DNA, he aspired to Clark's ideals. Clark was his role model; he wanted to be Superman someday. Without him, there's nobody to be Superman and nobody who would aspire to be Superman, so replacing Superman isn't viable.
Because Clark Kent is pretty much as iconic as superman is.
And there's no rule that says the stories of inconic characters can't end. The ending is a critical part of any story. Then, we can begin a new Superman story with a new Superman.
That said, there are arguments not to replace Superman. His potential for extremely long life, enduring into the future as a persistent beacon of hope and Christ-figure comes to mind--one constant in a changing universe. I haven't decided whether it's preferable to replace him or not yet. He has the option, anyway. Batman, on the other hand, is a symbol for the reality of human struggles--and that means he
must pass, as humans do. Moreover, his legacy
must endure beyond his passing because that is the impetus of the human race. Batman's story
cannot be whole without a successor.
The same with Bruce Wayne, and the same with Peter Parker. Their identities have become household names, everybody knows bruce wayne, peter parker and clark kent.
Again, I don't see why this is an issue. They'd eventually become familiar with the new names. We don't halt the story because Joe Blow won't immediately know about the change. We don't care about Joe Blow. Joe Blow doesn't buy comics, and therefore doesn't matter. Joe Blow just goes and sees the movies, which will feature Bruce Wayne anyway. And if the replacement is a success and in twenty years there's a movie with Dick Grayson as Batman, Joe Blow will see that too. He won't throw a fit that it's not Bruce Wayne because it's just another movie to him. He'll say "Well, that's new" and move on.
Comic book fans are the only ones who care about this stuff, and the reality is that sometimes they have to do things that scare the fans, lest we be reading the same stories over and over again for all our lives.