Right, you can't compare an origin film or solo movie to an established franchise that's putting together a dream team of superheroes. It's like those people who were comparing the box-office success of TDK or TDKR to Avengers.
Not really. There's certainly an audience for TDK and that trilogy as a whole, that watched those films because they had their feet planted firmly into a more serious crime drama type of genre. Rather than the bulk of comic book movies, sci-fi, "superhero dream teams" or anything that involved alien invasions, superpowers, etc. There's a great amount of people who don't watch ANY comic book movies yet they caught onto TDK-Trilogy because it was like watching some action-drama or a gangster film. Something like that. There's even a lot of filmmaker fans (whether that be of Nolan's or of directing in general) who would throw their nose up at Avengers yet they were interested in what Christopher Nolan, the guy who made Memento, had to say with a 200 million dollar budget.
So no, only a section of the people who saw TDK went on to see Avengers in the cinema automatically. A huge amount but I wouldn't say the majority.