That's Marvel for you. ^^
He is the crossover sensation but I'd say the situation is similar to how Superman remains the flagship character for DC while Batman is more popular across all media. I know Iron Man isn't at the Batman level but I wouldn't be surprised if that became the case eventually. Maybe Wolverine's next solo film does nearly as well but I somehow doubt that it will do Iron Man numbers.
As I sort of said earlier, I think that will depend on whether the main draw for the movies was actually the Iron Man character, or just Robert Downey Jr. potraying him. Batman is Batman regardless of who plays him... but with Iron Man at this point it's a bit less clear.
The movie Iron Man is a great character because he fundamentally did the same thing --- he also matured as a hero, a person, and a character throughout the course of the movies he appeared/starred in, growing from a shallow hedonist who could care less about the consequences of his inventions so long as he made money off them to maintain his extravagant lifestyle to someone who appreciated the responsibilities a man of his genius and stature had to the world and the people around him and strove to live up to them despite his failings in the past. That element of growth and progression and actual maturation shared between the movie Iron Man and most versions of Spider-Man is to me the major storytelling theme of the Marvel Universe and even shines through in some of the more recent ideas like the Avengers Academy, the Jean Grey School for Higher Learning, the Future Foundation, and even Spider-Man's ill-fated run with Alpha as his sidekick. To me, the overarching theme with those concepts is what this generation of heroes owes to the next generation, given that Civil War could possibly be interpreted as showing the worst possible outcome of when the older generation leaves the younger generation to its own devices without real guidance.
Alas, as others have noted here, Marvel's stubborn dedication to regression and stagnation in the case of characters like Spider-Man and Iron Man (undoing Peter's longstanding marriage to Mary Jane with One More Day, and "rebooting" Iron Man so that he's "no longer accountable" for his actions in Civil War and onward even though he went on to claim that he'd do the same things if given the chance to redo those events and later did even worse in the current New Avengers) puts the lie to that theme of progress and maturation I was just talking about, if that's what they're going to do with their flagship characters. It's really quite a shame.
Back in black, the hunter is ready to claim his prey.
"That's not just "one man"! That's TONY FREAKING STARK. You're intel should've warned us that he was James Bond and "Q" wrapped in the same guy!" Cobra
Reading: Deathnote, Batman by Morrison