Super-hero comics are a niche industry.
I know that may seem to be a funny thing to say. One need only to look at the monthly Diamond sales charts to see that super-hero comics from DC & Marvel dominate. But it's those same charts that give us a clue as to how small the super-hero comic market actually is.
Let's take a look at the current most popular super-hero comics, Marvel's Civil War mini-series. The latest issue for which we have sales figures is #5, where it moved about 273K copies. Now we know there were multiple cover incentives, and we also know that not every super-hero comics fan was buying Civil War; so let's make the math easy on ourselves and say, probably generously, that there are 300,000 people in North America who are interested in buying super-hero comics on an at least monthly basis.
Now 300,000 potential customers is nothing to sneeze at. If we assume that (again rounding to make the math easy) that there are 3000 comic book specialty stores in that market, that gives us 100 regular customers per store. Not huge, but of those customers are regulars who spend enough money you cna probably make a go of it. Of course you have to have a product that caters to those regular customers; from what we've seen over the past years, those customers like continuity-heavy, event-driven comics.
However, 300,000 is a very small number compared to the population: 300M in the US, plus another 33M in Canada. So we're talking less than one-tenth of one percent of the population is interested in regular super-hero comics. That is pretty much a standard definition of 'niche.'
I would argue that 300,000 was pretty much always been the upper limit of dedicated super-hero comics fans; even back in the early-90s heydey, when top super-hero comics could sell 1M-5M copies, it was gimick covers and speculators and collectors buying multiple copies pushing those numbers up.
300,000 is not, we should note, the upper limit of people who are interested in super-heroes themselves. The most popular super-hero movie of recent years, Spider-Man 2, took in $373,585,825 in domestic gross; if we assume $10 per ticket, that's 37 million people who wanted to watch a super-hero movie. Heroes, the #1 new television show of the season, gets nearly 14 million viewers per week.
Clearly people are interested in super-heroes, just not super-hero comics (at least as they present themselves).
300,000 is also not the number of people in North America interested in comics themselves. One need only to look at the bookstore market where teen-oriented manga and selected 'mainstream graphic novels' regularly outsell the super-hero collections quite handily.
There's nothing necessarily wrong with being a niche market. Most markets are. It means giving that market what it wants, which is why the Diamond sales charts are dominated by those types of comics.
One could make the argument (and many have) that, to break out of the niche to a larger market that is clearly interested in super-heroes, the publishers of super-hero comics should make super-hero comics that don't play to the continuity-heavy niche. The problem is that we've seen that those comics don't appeal to the niche, and thus cannot support themselves enough to survive in that ecosystem. And outside of the niche market you're competing with manga, and the manga kids have made it quite clear where they want to spend their comics dollars.
So super-hero comics are a niche, and most likely will continue to be a niche. It appeals to that niche market well enough to survive. I just don't see and clear way that it can break out; nor do I see that it necessarily should.