The history of movies is defined by twists on the status quo. Star Wars took the cerebral sci-fi of the seventies and turned it into space fantasy, Chinatown was such a competent throw-back to film noir it birthed a new type of movie, the neo-noir, and Psycho, with Alfred Hitchcock keen to push the limits of movie censorship, essentially created the whole concept of the slasher. That's an illustrious way to say that sub-genres splintering off from fully-fledged genres is a natural part of cinematic progression; something works and what made it work becomes popular. And there's nothing inherently wrong with that, helping things always keeping fresh. Heck, once things have started they can end up in exciting new places; superheroes are so prominent and distinct by themselves that they almost qualify as their own genre distinct from sci-fi. Not every sub-genre can keep things as fresh as those created by Lucas, Polanski or Hitchcock, however. And there are some (eight in fact) out there that are really doing nothing positive to the industry. Maybe they're once popular tropes boosted to their own sub-genre or an ill-thought out idea that for some crazy reason stuck, but whatever the cause they really need to just go away.
