Rocky Falls, Everyone Dies

In many bad B-movies of yesteryear, the writers had no idea how to end the film, so the trope Rocks Fall, Everyone Dies was born. Or it died. Or it did both simultaneously.

Because Rocky Horror is an homage to many of those tacky classics, it has a similar ending. Riff Raff reveals that he, Magenta, and Frank are aliens, and he proceeds to shoot everyone with a laser beam.

What a downer. Plus, the narrator concludes with this depressing analysis:

NARRATOR: And crawling on the planet's face, some insects, called the human race. Lost in time, and lost in space, and meaning.

Things got depressing really fast. Maybe this is why Magenta and Frank mention "The Time Warp" in their final dialog. They may be both space-travelling and time-travelling, but they also want to get that song stuck in your head so you don't think too long about how grim and hopeless the ending actually is.