I was watching the original Robert Downey Jr. "Iron Man" recently. The origin story was done very well, but you didn't have non-stop pointless action or lame villains the way that "Iron Man 2" did.
So, when I hear about a sequel for both Thor and Captain America, both of which were pretty lame in their first iterations, then a third Iron Man, a second Avengers, an Ant-Man movie, plus a movie for a more obscure group of heroes called the Guardians of the Galaxy which include a raccoon and a talking tree, it seems to me there's a growing bubble of inflated expectations for Iron Man and the many related offshoots of the Marvel Universe.
If you aren't familiar with the idea of a "peak," here's a post about the most famous: peak oil.
This is not particularly surprising, given how poorly thought out the Batman iteration was in the '90s, which finally ended in the abomination of "Batman + Robin," which hardly any one ever brings up to George Clooney now (and also may have been "peak Chris O'Donnell," an actor who people may tend to forget was a pretty big deal in that decade, then didn't do a single movie for four years and is now mostly known for an NCIS spinoff.)
In the same vein, the "Fantastic Four" movies seemed to flame out pretty quickly, with the second movie being horrible. The Matrix trilogy also comes to mind.
At one point last year, there were literally four Iron Man-related cartoons on basic cable. There are still two that remain, and an animated Avengers reboot that is already in the works. There was even talk about doing another release of Avengers in some kind of Director's Cut because $600 million wasn't enough for the studio.
To some extent, you see the same thing with the Wolverine character and the X-Men, where no one is particularly interested in another X-Men movie about any one except Hugh Jackman's character, although there is supposedly a "First Class" sequel in the works. There were also three Wolverine-related cartoons last year, none of which were particularly successful.
At this year's Comic Con in San Diego, Robert Downey Jr. seemed to intimate that "Iron Man 3" will likely be the last in the current run. But, right now, there seems to be no limit to the potential prequels and sequels, some of which could even be done direct to video.
But I wouldn't be surprised if either one or all of these movies under-perform at some point, especially after the big budget flops of "Green Lantern," "Battleship" and "John Carter." At a minimum, there has to be diminishing returns.
This post at IO9 essentially makes a related point, that there are only so many viable super heroes, and Guardians of the Galaxy likely aren't one of them: There are only a half dozen A-list superheroes.