Most would agree that we are in the golden age of comic book films. With a new movie coming out every few months some believe that there will be "superhero fatigue" from over saturation, similar to western which I think hasn't actually gone away, see Hateful eight or Hell or High Water. I don't see fatigue I see growing popularity and a sustainable source of content. In 2016 there were 7 movies based on mainstream comics: Deadpool, Batman V Superman: Dawn of Justice, Captain America: Civil War, X-Men Apocalypse, Teenage Mutant Ninja Turtles: Out of the Shadows, Suicide Squad, and Doctor Strange and there are 8 scheduled for released in 2017.
With the 7 movies last year making over 5 billion dollars total at the box office with 4 making more than 700 million dollars worldwide, I don't see dwindling audiences. Also, I don't see studios willing to walk away from that kind of cash cow. Both DC and Marvel the two largest comic book publishers have released their slate of films for the next 4 years and there over 40 movies in development. If they all get made that would be around 10 movies a year or almost 1 a month.
Tentative slate of upcoming films
As I mentioned earlier there is an audience for these films. Doctor Strange who is one of the flagship characters for Marvel Comics brought in over 650 million dollars. In terms of popularity, his name isn't on par with Batman, Superman, and Spider-Man but it found a niche being visually stunning and telling a type of story that hasn't been told for comic books in live-action form, not to mention it is part of the highly successful Marvel Cinematic Universe or MCU. The MCU which many credits with the current superhero boom, has grossed 10.7 billion dollars with a total of 14 movies, with 2012's Avengers being the highest grossing and 5th all-time at over 1.5 billion. Despite the box office success, all the movies haven't been critically acclaimed.
Types of "Super Hero" Films
Of course in all types of movies there are films that don't do well commercially or critically or both, see 2015's Fantastic Four. A lot of those saying there is or will be superhero fatigue use that film's failure as and argument. Reboots are also a contributing cause to the belief of superhero fatigue. There have been three different actors to play Spider-Man in the last 10 years. Movie fatigue only seems to be associated with superhero films. Other genres of movies like horror, comedy, drama or animation are never called into question. There were more animated movies released in 2016 than superhero movies but there aren't talks of animation fatigue.
The reason, in my opinion, is because Kubo and the Two Strings is not the same type of film as Zootopia both of which were critically acclaimed animated movies that opened in 2016. I believe the phrase "superhero fatigue" is too broad. In the "superhero" genre there are many different types of films. The Dark Knight and Guardians of the Galaxy's only similarity is that they are characters from a comic book. 2008's The Dark Knight is a crime drama. 2014's Captain America: Winter Soldier is a spy thriller. 2014's Guardians of the Galaxy is an intergalactic Sci-Fi movie. 2015's Ant-Man is a heist action Comedy. 2016's Doctor Strange is a fantasy adventure film.
Of course, you get the team-ups that have the "do the right thing save the world" plot with movies like The Avengers and Justice League but overall there is diversity in the storytelling of comic films. This is mainly due to the variety of stories and characters that they draw from. The modern superhero goes back nearly 80 years to Superman debuting in Action Comics #1 June 1938. Over time many writers have lent their talents to tell intriguing and captivating stories to comics giving studios plenty of source material.
Of the 8 movies being released in 2017, you have Logan which looks like a post-apocalyptic, survival western. Spider-Man: Homecoming looks like a coming of age film. As well as Wonder Woman which is part war movie part adventure/ Greek mythology film. As with any genre of movies when the writing and plots become stale or repetitive there will be a lack of interest. Also as with anything, there are cycles but the diversity of superhero films and the advancement in VFX will allow for creators to accurately portray these stories. Superheroes movies may not always dominate the film landscape but in order for people to become tired of them, the population would have to grow tired of movies in general because comic book movies encapsulate all genres.