Well, I didn't read the entire rant there, but here's my thoughts. First off, the reason superheroes dominate the comic format is because they essentially created it, along with smaller newspaper comics, which are still around too, though I guess that's not technically the same format. And also because they still sell. It's not really a big secret.
Superheroes were not the format that created comics. In fact, they were somewhat late to the game and one of the last genres created. Westerns, crime comics, romance and war stories, along with a variety of "funnies" were around long before Superman. And it's not that superhero comics outsold the other genres, it's that the other genres were gutted in the 1950s after Dr. Frederic Wertham published his indictment of comics, Seduction of the Innocent, which convinced parents and lawmakers that comics were destructive influences on children. A very few superhero titles (pretty much only Superman, Wonder Woman and Batman) barely survived and the industry rebuilt on the superhero genre because, under the restrictive comics code authority, it could be presented as wholesome and palatable for children. And to say they "still sell" is to ignore that their sales have been steadily dropping for over 20 years. Manga, on the other hand, which is largely fantasy and romance and sells to adolescent girls for the most part, has been blowing off the shelves for 15 years with steadily rising sales. That's the reality of which genres sell.