You can't use the history of prior comic movies to predict the future of Marvel's current run because their approach has been decidedly atypical.
Prior to The Avengers the prevailing logic was that you first make the team movie (or a solo movie with a guest character) and then spin off a new franchise. Marvel established a precedent by doing it the other way around.
The typical Hollywood studio model is built upon formulas to maximize the opening weekend. To that end, a typical blockbuster is constructed around the formula of big stars, big effects and a killer trailer. If a film like, say, Donner's Superman, was a smash hit then the sequel would be a guaranteed opening draw. With that in mind, why dump the same budget into the sequel? Let's cut out Brando. Let's rush the production. Let's cut the effects budget. The dumb fans will come to opening weekend regardless of how good or bad the film is. That's how franchises often die.
Raime's Spider-Man run was crushed in the 3rd film by the studios with creative differences over the content. Studios wanted Venom because he was popular. Director didn't. You saw the result.
But, it seems, Marvel doesn't crap out on the sequels. They're not about cranking out the obligatory sequel but trust the source material to guide the evolving stories organically.
Marvel's Kevin Feige's difference is that he believes in translating the spirit of what he loved in the comics to the screen. That's all. This is also generally true of the directors they pick - an appreciation of the comic books themselves. The Hollywood studio "rules" don't apply. That's a fundamental difference with the Marvel films. He's not following a "formula." He and his creative teams and directors only want to bring their favorite stories to the screen.
After The Avengers, the public discussed whether Marvel or Warner/DC be the first to bring a female superhero movie to the screen. Many folks wanted Marvel to make a Black Widow film. But Marvel's plans weren't dictated by serving a demographic and no Widow film was planned - instead Feige just said the schedule of films was mainly dictated by the long story being told. Warner/DC, on the other hand was quick to be the first to announce an upcoming female superhero film Wonder Woman (with female director) and black superhero film Cyborg at a shareholders meeting. Marvel, on the other hand, held a special press event at El Capitan about their "Phase 3" plans for fans (not shareholders).
It feels like Warner/DC is still trying to anticipate the market of public interest instead of just making movies that comic readers love. I feel like Warner/DC's folly is the Warner exec component. Marvel has shown that, if you want to make watchable comic films with lasting appeal, you need to have them made by people who love and understand comics. If Warner actually entrusted creative control of their superhero films to the DC Comics division I think they cold do something really special.
I feel like, as long as Marvel has new stories to tell, they'll keep making movies and they'll still have an audience. It's when films become formulaic and redundant that the audience dies. If the audience shrinks I think Marvel will work on smaller scales, on lesser budgets, focus on Netflix series but will not stop as long as they're backed by Disney and have tales to tell.
Maybe the analogy with westerns is apt. Even with the demise of the boom in western films - there are modern westerns (Unforgiven, Lonesome Dove, Silverado, Tombstone, Django Unchained ...) that arise, not as a result of anticipated public interest in the genre, but in the need to tell a good story.
