The subject of "Too many Superheroes" is one that's been debated for a good few years now. Recently Steven Spielberg himself chipped in on the discussion saying that they will surely go the same way the Western did, i.e. have a decade or too of insane popularity and then rapidly fade as everyone got sick of them. Zack Snyder also responded to the argument. From a general perspective, Spielberg has a point. There are many similarities to the two brackets of film (one being my insane level of adoration for them) so it makes sense that Superheroes will go down the same road, right? Well, while I'm not discounting it as a possible outcome, there are some subtleties that Spielberg may not have considered which could make all the difference.
1) It's not a genre
A pet peeve of mine is when comic-book films get called a genre. They aren't and never will be, because comics and the films adapted from them cover basically all the genres out there. If you want horror, action, drama, romance, spies, sci-fi, comedy or any other kind of film there is, then there's a comic out there for you. Hence the movies based on these stories can fall under any category. However, westerns are, in and of themselves, a genre. They have a specific setting: the Wild West, follow a specific kind of person: the square-jawed pinnacle of masculinity, all have a similar tone and can only pick from a handful of themes: becoming obsolete, vengeance, war etc. Contrast this to the Superhero films we have. There are crime thrillers in The Dark Knight, identity crises in Spider-Man 2, redemption in Iron Man, questions about heroism in Watchmen and the list goes on. There is a plethora of different beats you can hit with superheroes; from Blade cutting a bloody swathe through vampires, to Wonder Woman taking on Hercules, to Dredd violently and brutally taking down mobs and the undead to Daredevil slowly piecing together the clues of a murder. Westerns simply didn't have that diversity.
2) World War II
Part of the reason that Westerns took off the way they did was because they offered audiences a black and white story of classic Hollywood style masculinity. The men shot each other over nobility, the good guys won and the cowboy got the beautiful gal. During a time where war was so prominent in people's memories and tearing lives apart, people flocked to seek some comfort in the good old days when war wasn't such an ugly thing and people settled differences mano a mano instead of by sending their youth to squat in muddy trenches and suffer through an endless nightmare. Thus, when the world began to move on from Second World War ended, Westerns quickly declined as the masses finished rebuilding and tried to begin a new era. However, superheroes are popular simply because of their broad appeal and simple way of telling stories that resonate by blowing them up to a gargantuan scale. There isn't any correlation between superheroes' popularity and world events, they're just good movies. Granted, so were the Westerns for the most part, but they were fulfilling a need that was destined to pass away whereas superheroes simply entertain us on a fundamental level.
This... seems like a weird one when you put it into one word like that, but allow me to expand upon it. It's inherent to the Western that it takes place in the Wild West, after all, it's only in the bloody name of the genre. That means that the dozens upon dozens of the most popular film genre in the 50s were all taking place in the exact same setting. Wouldn't that get a little dull? Imagine if all the Superhero films took place in New York? Wait a minute, that's a bad example. But speaking honestly, superheroes can go anywhere: any of the five continents, fictional cities, the past, the future, space, alternate dimensions and universes and pretty much any other place a person can dream up. So the films the superhero machine cranks out can all look different and thus feel different, allowing them to seem fresher and less similar.
4) The comics
Ever since Superman's debut in 1938, superhero comics have been going. Sure, they've had their ups and downs, but icons like Batman, Superman, Spider-Man et al have all managed to stay relevant, and keep themselves with prominent comic titles for as much as 75 years. Yet, some of the stories being told today are still doing new things with the characters and telling some of the best stories yet. For example, in the past few years Scott Snyder and Greg Capullo have put together one of the best Batman stories of all time if you ask me. So why can't a similar thing happen with the movies? If the comics can update themselves and continuously deliver poignant stories for decades upon decades, then why can't the films? They even have the vast treasures of the existing comics to tap into for inspiration. It's basically a template to work off of when creating your worlds, so use it, and maybe superheroes will one day be viewed as simply a part of cinema as much a spy films or rom-coms.