I would argue this is not so much the rise and decline of geek culture, but the rise and decline of comic book superhero culture. Science fiction, fantasy and horror have all been fairly successful genres of film and television since at least the 1950s, and especially since the mid-70s. They go through cycles of popularity and obscurity as well, but tend to bounce back in some way, even if it's a new subgenre. Inception is by any definition of the word, a science-fiction film, and it was immensely popular, and wasn't based on a comic book or video game. It's like Nolan made a cerebral sci-fi film of ideas from the 1970s, but with modern special effects and good acting.
The past 10-15 years have been the first real successful iteration of superhero stories in mainstream film after so many fits and starts. Partly because the technology is finally there to render them, and partly because filmmakers who grew up with them are now making movies. But it was going to end eventually, because superhero stories have limited appeal.
Why? Bob brought this up tangentially in one of his Game Overthinker videos, but basically comic books have been catering to the same 25-50 year old male demographic since the mid-80s, and indulging way too much in the "grim and gritty" anti-hero stories of the 1990s Dark Age. These are not how you cultivate the next generation of comic book fans. Luckily, the current and upcoming class of comics creators seem to get it, most recently and notoriously, Darwyn "stop catering to the perverted needs of 45 year old men" Cooke [http://www.youtube.com/watch?v=xgMZl0FJsx4], but also the Marvel Adventures line, the more recent DCU animated adaptations, and so on. So even if the current trend for capes peters out in the next year or so, hopefully we'll see it rise again in another decade when today's preteen fans become the media tastemakers.