Okay, I'm just thinking out loud here.
Disney's newest Marvel push is undoubtedly to the stratospheric success of the Avengers, a movie which I would consider the high point of a newly-formed superhero genre. I mean, let's face it. With multiple superhero films coming out a year now, it has effectively become its own genre.
Let's look at the last time a big new genre hit Hollywood and took America by storm: the Westerns of the '50s and '60s. They were huge for a while - heck, for two decades - and even earned coveted placement in Disneyland in the form of Frontierland. Walt dramatically expanded Frontierland to capitalize on the public's interest in this new style of movie. Davy Crockett was the hottest character in the park and no kid under 10 would be caught in Disneyland without a coonskin hat on his head.
And then it ended. Hollywood moved on, the Western wore itself out. The occasional modern Western (lookin' at you, Wild Wild West) no longer captivates America like the movies of old did. And Frontierland went from being Disneyland #1 destination to being that one land with the roller coaster, wedged between Fantasyland and New Orleans Square.
Noticing a parallel here? Sure superhero movies are popular now... but how long can it last? Almost of the superhero films being made are adapted from characters that are decades old. Does anyone think Spiderman's 9th sequel, Iron Man's 7th sequel, or the 6th Avengers movie will still hold public interest like it does now?
So what if in the 1950s, Disney had built an entire park dedicated to Western movies? You have Shane-land over here, High Noon-land over there, and be sure to stop by Magnificent Seven-land on the way out. I'm sure it would have been extremely popular... for a few years. But there's no way that such a park would have a following today without serious replacements on the order of an almost entirely new theme park.
Disneyland played it smarter. The park toned down the emphasis on branded franchises, and chose not to focus on a single genre (as it could have so easily done with Westerns, or Fantasyland-style Disney movies). Instead, it featured lands representing multiple genres of fiction, so lands could swell, shrink, or be rethemed as genres went out of style.
To sum it up, I think an entire park based on Marvel is a terribly short-sighted move. Even a new park in which 3 out of 6 themed lands were Marvel-themed would be vastly preferable, so the park would have some sort of protection from dating itself too quickly. I hope Disney has the foresight to make that decision.