Will mainstream success kill San Diego’s annual comic book and sci-fi convergence?

Will mainstream fame destroy Comic-Con? Will the nerds succumb to the narcissism enabled by too much Hollywood love? Will the geek raison d’etre be diluted as the films that fill the hallowed halls of the San Diego Convention Center starting Thursday drift further and further from the comic book, sci-fi fanboy core?

I hope not, but there are signs that erosion is already well under way. So I do have fears. And so should you. If the obsession and passion of the freaks and geeks soften, something precious will be lost to all of us. Seriously.

For the last decade or so of its 41 years, Comic-Con came to represent a special strain of movie fan: the ones who cared far more about the material itself than the material gain possible if a comic book franchise such as “Spider-Man,” “Iron Man,” “Batman” and just about any other sort of fantastical “-man” (even the occasional “wo-man”) took hold.

They didn’t care how acclaimed the director, how bright the star, if you didn’t measure up to their very high bar, they would flay you. When the Hollywood power brokers started showing up in the ’90s, the hardcore fans who had been making the trek since the ’70s and ’80s, when Comic-Con was little more than boys with their boxes of comics to trade, were unimpressed, unintimidated.

As a result, it was the one place to find truth-tellers who would shout their likes and dislikes loud, long and on the record. Cast Halle Berry as “Catwoman” in the wake of her Oscar win for “Monsters Ball” and the screams would become howls that sent chills down the studio spine, as the Los Angeles Times’ own super geek, Geoff Boucher, found in his analysis of the Comic-Con effect in 2005. The film and its star went on to pile up a ton of Razzies (the annual award for the worst this and that).

With the power of the fanboys finally registering — they could make or break a film — in typical fashion, there were those who wanted to harness that power. In the kingdom of the geek, no longer would sending a trailer suffice, as Tim Burton was able to do with “Batman.” The actors, directors, producers and marketers were expected to show up in person to kiss the ring. And before you could say, “Ching, ching,” the convention had blown up into a giant focus group, featured in real time on the Internet.

But a funny thing happened on the way to the megaplexes and the IMAXes — the formula stopped working. Last year, the buzz out of Comic-Con for “Jonah Hex” was hot. When it opened in June, it quickly flamed out. In 2008, the C-Cers loved what they were seeing of “Watchmen.” Its $55-million opening in 2009 wasn’t horrific, but it wasn’t close to expectations. The problem with “Watchmen” was that it hewed too closely to the source material; bowing to the “be-true-to-the-source” creed of the Comic-Con crowd (whether consciously or not) was its undoing. And the hex on “Hex” just proved the notion that anyone could be fooled by a cool trailer.

A box-office failure was one thing when a movie hadn’t gotten that Geek Housekeeping Stamp of Approval. The studios had learned to just take their lumps — “Elektra,” anyone? But what were they to do when Comic-Con hotness didn’t hold when the film was released?

The problem, as is so often the case, is that success, unless it’s handled carefully, can spoil, just as power can corrupt. As the Comic-Con throngs went from 50,000 to 100,000, fanboy fanatics were increasingly being replaced — or at least outnumbered — by the fawning multitudes. The “cool” kids started coming in droves as the studios ramped up red carpet rollouts and the star power became blinding.

What was once a tight consensus of true believers was becoming a flabby behemoth of mixed opinions that couldn’t be trusted. Sure, they picked “Toy Story 3” as a winner, but frankly, the kids down the street running the lemonade stand could have done that and for a whole lot less trouble.

For now, Comic-Con retains its power against all comers (a few have tried to siphon it off its power to little avail; Orlando, Fla.’s MegaCon, Austin, Texas’ Butt-Numb-a-Thon and even the Comic-Con spinoff WonderCon in San Francisco haven’t made a dent).

Hollywood continues to come calling in ever bigger numbers, lugging more the sort of mainstream movie baggage that would have been rousted and rejected by the faithful a few years back. “Salt,” starring Angelina Jolie, might turn out to be a terrific spy thriller, and “Tangled,” Disney’s coming Rapunzel adventure, might be great fun, but worthy of Comic-Con cred? I don’t think so.

The perception of the convention as still belonging to the geeks lingers, while the reality is that the cool kids are taking over. Before that happens, I am hoping for a coup d’etat (coup d’geek?). Whatever it takes to get the purity back, return the sword to the stone, the rings to the lord. Otherwise, I fear we’re in for years of bad casting, bad sequels and bad things being done to swashbuckling sci-borgs and comic book heroes who deserve a far better fate, as do we.