Even though certain Batman comics are considered some of the best of the comic book medium, they do not have the fanbase that certain franchises like Harry Potter has (I feel like I've said that a million times). The difference between the Harry Potter franchise and the Batman franchise is this: Harry Potter has a large cast of characters and storylines that everyone knows. Batman has a large cast of characters that most don't know. If you ask 100 people on the street who Professor Umbridge is, you will get a better response than if you ask 100 people who Clayface is.
You're purposefully making it an unfair comparison. I'll do you one better. Get your top 5 or 10 Potter characters, and match 'em up with these guys:
Batman
Alfred
Joker
Penguin
Catwoman
Two-Face
Riddler
Robin
Batgirl
Commissioner Gordon
Guaranteed if you poll whatever random sample from the street, a significantly larger percentage will recognize those names than they do Potter's.
And the fact that Batman has 70 years of stories to draw from isn't exactly a plus. Over those 70 years, nearly all of Batman's cast of characters has changed significantly, whereas Harry Potter's cast haven't changed much at all.
The comparisons aren't valid. You're comparing 70 years of literature with 10. Moreover, my reference of the vast history is only to point out that the source material is able to provide a number of stories to take inspiration from. It's only fair considering you wanted to point out the 7 Potter books.
Not only that, but there is no canon storyline that Batman clings to thanks to numerous retcons throughout the Dark Knight's 7 decade history. You can even look at other Batman media as an example. Of the numerous actors to play Batman over the years, very few of them have had similar portrayals of the title character. Even more dramatic is with the Joker. Cesar Romero's Joker is the polar opposite of Heath Ledger's, and Jack Nicholson's and Mark Hamill's are both somewhere in between, but still not similar to each other at all.
What does have that have to do with anything? None of the batfilms have directly adapted a specific story. It's not a necessity to make a cinematic venture. TDK made a billion dollars off an original story. I'd say they're fine.
To sum up my point, Harry Potter has a set, static storyline, whereas Batman's is fluid and always changing. Therefore, the producers of Batman films would have to come up with new storylines for every movie done, whereas the producers of Harry Potter would simply have to decide what to cut from their source material.
Bond is on it's 23rd film. I don't need to point out how lucrative that franchise
still is. Batman will be on it's 8th live-action movie, the last of which broke several records. Like I said, longevity isn't an issue here. If it was so difficult to churn original stories that the crowd reacts to, the character wouldn't be as prominent today.
Typical how people will just come to these boards and bash someone for posting an idea and saying how dumb it is and not giving a reason for why it is so. Yes splitting a film into two movies hasn't always worked in the past, ex. matrix and pirates. But those films were split mainly to just get another movie out there and make more money. And if that's what Warner would be trying to do then yes it would be a bad idea. But it did work in the case of kill bill, and they split that film just because the story was just too long and complex for one film.
I guarantee everyone would embrace the idea of splitting batman 3 into two films if Warner Bros announced that batman 3 and 4 would be filmed back to back and directed by chris nolan and then later nolan explains that, "David Goyer and I have come up a story that we are very excited about and the complexity of the story will force a very long runtime. Therefore we have decided in order to tell this story properly we will split the film into two parts." If Nolan said something along those lines I bet you wouldn't think it was a dumb idea. I know I would definitely be excited.
But lets not forget that Goyer originally planned for the Dark Knight's story to span over two films but they crammed it all into one and it turned out great so I highly doubt it will happen this time. But I certainly wouldnt mind if they did this (for the right reason of course).
Yeah, it's really as simple as that. Some stories are just too big for one movie. As I said originally, if the scope and narrative calls for it, why not? I don't get the sudden backlash towards the idea. Nolan would have you all eating out of his palms if he presented the announcement himself.