At last week's Comic-Con event in San Diego, the film director Zack Snyder bounded on stage to announce a bold new merger. His next Warner Bros blockbuster would pit Batman against Superman, two costumed superheroes in one movie. "Let's face it," said Snyder, "this is beyond mythological." The fanbase was galvanised. Hyperbole hit the roof. But in Hollywood, alarm bells were ringing.

Industry insiders are referring to this season as "the summer of doom" – an overcrowded huddle of big-budget spectaculars, without the audience to sustain them. US box office takings are down 19% on the same period last year, while the studios are smarting from such high-profile casualties as The Lone Ranger, After Earth and the supernatural action-thriller RIPD. While the runaway success of Iron Man 3 and Despicable Me 2 helped soften the blow, major figures claim that the industry needs to adapt quickly or die.

Speaking on a panel at the University of Southern California last month, the film-makers George Lucas and Steven Spielberg suggested that the era of the $300m movie dinosaur may well have run its course. "There's eventually going to be a big meltdown," Spielberg said. "There's going to be an implosion where three or four or maybe even half a dozen of these mega-budgeted movies go crashing into the ground – and that's going to change the paradigm."

Under Hollywood's current business model, nothing succeeds like excess. Prevailing wisdom stipulates that action movies or established franchises – such as Transformers, Pirates of the Caribbean or superhero movies – come with a pre-sold fanbase and vast potential for spinoff merchandise.

It makes more sense for studios to spend $300m on a summer blockbuster than $50m on original mid-range pictures that must then be marketed to audiences. The larger the film, the larger the return. "Go big or go extinct," is the motto in Pacific Rim, a Warner Bros production in which giant monsters battle giant robots. And yet Pacific Rim is already being filed as another casualty of the summer of doom.

"It's unlikely that the studios are going to drastically change course as the result of one bad summer," said Steven Gaydos, executive editor at Variety magazine. "However, it is imperative they diversify their slate. They're laying down too many big bets without anything else on the agenda. They have to kick their dependency on $300m blockbusters. If they don't, they're going out of business."

Yet Gaydos cautioned that the situation was complicated by the opaque nature of studio accounting, in which the US box office tally is just the tip of the iceberg. "Take a film like Pixar's Cars. It made so much money from bedsheets, towels and coffee mugs that the movie is essentially a commercial for the toys." Action blockbusters such as Pacific Rim, which have merchandise potential and a decent overseas audience, may not turn out to be the box-office disaster that they first appear.

Despite stormy forecasts, Hollywood appears to be too unwieldly or too unwilling to shift direction towards smaller, cheaper pictures. Guests at Comic-Con learned about upcoming studio productions including Pirates of the Caribbean 5, Thor 2, Fantastic Four 3 and a reboot of Godzilla. The director Joss Whedon came to the event to lament that "pop culture is eating itself" and called for "new universes, new messages and new icons". He then revealed the title of his next film to be Avengers: Age of Ultron.

"Look at Comic-Con and then tell me if you think Hollywood is going to cut back on its comic-book dependency," said Gaydos. "Look at how that event was covered by the critical establishment and you'll see how everything still validates the conglomerates' bottom line. By and large, people are not looking for intelligent, edgy, mid-range movies. They're looking for superheroes and special effects. They're looking for amusement rides. They're like the kids in Pinocchio who still want to go to Pleasure Island. They're voting to be donkeys."