Movie genres run out of steam. It has ever been thus. So many Westerns were made in the first half-century of the American cinema that they still outnumber any other genre, but they're a footnote now. Gangster movies were all the rage in the 1930s, films noir in the late 1940s, musicals from the Depression to the swinging '60s, and teen comedies in the 1980s and '90s. They seem to settle in forever… and then they fade. So too with the dominant genre of the 21st century, the superhero picture.