Let's do a quick experiment. Picture your favourite comic book Super-Hero team up. Not the one from a ten year old cartoon, or the modern cinema, but a classic one. The classic Justice League, the Avengers, the X-Men. The classic line-up for the Justice League was Batman, Superman, Wonder Woman, Flash, AquaMan, Martian Manhunter, and Green Lantern (Hal Jordan). The Avengers have always been a little bit more fluid, but their core line-up has typically included Iron Man, Captain America, Thor, Hulk, Ant-Man, Hawkeye, and Spider-Man, with perhaps Ms. Marvel and/or Black Widow, and Wolverine. The classic X-Men line-up is Professor X, Cyclops, Jean, Wolverine, Storm, and Beast. All of these teams have varied a little bit, and this isn't a complete listing, but it'll do.
What do all of these people have in common? These are probably the 22 best known super heroes. And of them, there are five women (six if you count Hawkeye), and one non-white person (who counts double, Storm). And, before you get on me for the Martian Manhunter, his alter ego, John Jones, is a white guy, Phil Morris's interpretation not withstanding.
It's no secret that comic books have typically catered to a certain demographic. During the Golden Age of comics, the only time someone who wasn't white appeared was as a villain, and women were objects to be rescued. Even during the Silver Age, DC's greatest super heroine was relegated to the status of secretary.
In order to become more mainstream, comic books need an influx of diversity. This can be done in basically three ways.
Method One: Create an All New Character
You know those annoying Facebook Quizzes? Bet we can guess your age in ten easy steps? Here's one for you. Who's the Green Lantern?
If you said "Hal Jordan," you're probably 35 plus. If you said Alan Scott, you're probably trying to figure out how the magic box works. For everyone else, one of two names come to mind. Kyle Rayner or John Stewart. (And if you said "Guy Gardner," or "Abin Sur," you're just being difficult). John Stewart was a relatively minor character introduced in the 70s. His early introduction is incredibly different from how most of us know him. He was a walking, talking stereotype, and it was his later inclusion in the Justice League cartoon which elevated him to "the" Green Lantern many of us know today -- to the point where many people were upset that they "replaced" him with Ryan Reynolds in the Green Lantern film. Ironically, John was not the original Green Lantern introduced into the DC Animated Universe (DCAU). Kyle Rayner was introduced in Superman, in a possible back-door pilot. His omission from the Justice League was left unexplained for three seasons, until a random appearance.
John Stewart was picked for the Justice League cartoon because the producers wanted to introduce some diversity to the group, to appeal to a broader audience. This is also why Hawkgirl appeared, rather than Hawkman or Aquaman.
This is also why John Stewart was created. However, creativity seems to be at a low ebbing right now, and no one is creating anything new. Just rebooting. Creating an all new character is risky -- especially when there's so much that has already been done. It's hard to make your brand new character stand out, and you have to gamble on its popularity.
Method Two: Reboot Your Character
This method is much more limited than that of simply creating a new character. Instead of creating an all new character, the next time a character is rebooted, which seems to happen fairly frequently, the character is rebooted as something new. One example of this was the recent reboot of Green Lantern, Alan Scott.
The original Green Lantern, Alan essentially faded away until the New 52. He's never featured prominently in any of the major shows or movies, and is seldom even honoured with a quick "blink and you missed it" cameo. However, he leapt to infamy when, in 2012, his rebooted form became gay.
This was also done with Iceman, Bobby Drake. In a time travelling based reboot, Bobby realises that his older self is a deeply closeted gay man, and chooses to not go down that path, and accepts himself.
This method is severely limited. The sexuality of a character can be reimagined, but it is fairly difficult to reboot Superman from the son of white farmers in Kansas to the son of Hispanic migrant workers -- not without making him Zod's son instead.
This also creates a huge continuity issue. If you're only familiar with the "new" version of a character, you'd be deeply confused to read an old issue where Alan Scott's wife and children appear. Personally, I think it diminishes a character to suddenly change part of their background.
Method Three: Pass the Torch
The final method, and the one being embraced by Marvel, is a passing of the torch. Your traditional superhero, who has vanquished his foes for fifty years or more, finally says, "I'm a product of the 50s and 60s. My time has come. It's a new era. It's your turn," and passes his mantle onto a new hero.
This is being done a lot. Someone else became more worthy of Mjollnir and it's power, and a new Thor was born. Peter Parker passed his mantle to Miles. Steve Rogers sighed a long overdue breath and handed his shield to Sam. And, in the most recent change, Bruce Banner passed his Gamma Ray Regrets on to Amadeus Cho. So, we replace four superheros with a woman, a black man, a Hispanic boy, and a Korean boy. We still have a Thor, a Spider-Man, a Cap, and a Hulk, but they're new. They represent more diversity, and hopefully a better reflection not only of America, but of comic book audiences. And, because they're all new characters, they can have all new adventures, rather than just re-reading the same stories, told again for new audiences. Look, we know how Tony and Steve are going to interact, no matter how many times you reboot them or what era we put them in. We don't know what Tony and Sam will do. Or Tony and Amadeus.
Passing the mantle on to new characters is not only realistic, as it allows our traditional heroes a chance to grow, mature, and age, but also brings in much needed diversity, which will hopefully attract new readers. That's not a bad thing.