So now we know. Springfield, Vermont, has been named official home of The Simpsons. For this month, that is. The Simpsons Movie, it has just been announced, will get its world premiere there on July 21. The Vermont venue beat 13 other identically named US towns in the competition to host the event, having had to prove how similar they were to the fictional Springfield inhabited by America's number one animated family. Vermont citizens clinched the prize with their own video, in which a Homer lookalike gets pursued through the streets by a giant runaway pink doughnut. Having a nuclear plant nearby no doubt helped the town's bid.

It's the kind of stunt that would fit perfectly into the show. Which is precisely the problem. The Simpsons of today revels in big, stupid antics, one-note gags and obvious plot twists. The Simpsons of yesteryear, however, was a different beast, one that would have found no room for over-sized pastries pursuing characters along sidewalks. That's why it's hard to greet the arrival of the movie with whoops of excitement. If it's anything like the current TV show, this will be one of the greatest misfires in spin-off history.

You can almost hear the panic in the voice of The Simpsons' creator Matt Groening. The film will be "deliberately imperfect". It contains "everything we couldn't show on television". His co-producer Al Jean has even boasted that "if you've never heard of The Simpsons, you can enjoy the film". They know expectation is sky-high, even for something that's been 15 years (yes, 15!) in the pipeline. So why the need to qualify the film with so many caveats and premature apologies? Could it be that they know, deep down, The Simpsons is but a shade of what it used to be?

Once, it was the greatest show on TV. Every episode was brimming with imagination, excitement and some of the sharpest one-liners to come out of America for decades. But above all it was smart: The Simpsons knew how to parry crudity with intelligence blow for blow. Bart's big-haired nemesis Sideshow Bob stepping on a rake nine times would be followed up with a surreal two-minute performance of HMS Pinafore. Homer lobbing a lookalike of himself over a waterfall would be followed by a reference to Walt Whitman's collection of poems, Leaves of Grass. This was dizzyingly intelligent, daring, exhilarating stuff. For every burp gag came an arch pop-culture reference. For every time Homer fell down the stairs or Bart got strangled, we had a nifty TV parody or sly political dig.

And it kept on coming, week after week. An entire generation didn't understand it. George Bush senior, then US president, even wished aloud that American families could be more like the Waltons than the Simpsons. A massive rift opened up between those who "got" The Simpsons and those who hated it. You chose your side carefully. To be a Simpsons fan was truly one of the most privileged things in the world.

Then it all changed. A new guard took over and ripped up the rules. Veterans of the show with pedigrees on venerated US comedy institutions like Saturday Night Live and The Tonight Show - Jon Vitti, George Meyer, John Schwartzwelder - either departed or went part-time. In came writers who had cut their teeth on sappy teen comedies like Blossom and unsophisticated knockabouts like Beavis and Butt-Head. A looser, lazier sensibility took hold, given free rein by new executive producer Mike Scully. And the show became stupid.

You can even put a date on it: 1997, in the early episodes of the ninth series, where the head of Bart's school, Principal Skinner, was suddenly, arbitrarily revealed to be an impostor, and his entire life to date had been a lie. Come again? A major character in a long-running series gets unmasked as a fraud? It was cheap, idle storytelling.

This was just the start. The show went on to jettison all interest in pretending to have earthy, avuncular roots: the warm, good-natured centre that, when you scraped away the multi-layered jokes and cerebral grandstanding, had been there from day one was obliterated. No longer did we see the family bonding, caring for each other, showing emotion. Instead, it was anything goes.

Plots swung sickeningly from one cliche to another. Jokes arrived out of the blue for no reason. No attempt was made to cling to reality. Now Homer would end up in new employment six or seven times a series. To date, he's held 118 (and counting) jobs, from missionary to garbage commissioner to grease salesman to fortune cookie writer, which wouldn't be such a damning statistic had almost none of them been particularly funny.

True, a long-running series has to evolve. Nobody would expect Simpsons episodes to still be solely about Lisa getting a pony or Bart failing a school exam. But, in the second decade of its life, The Simpsons evolved into a dreadfully predictable monster. With each new series came the same questions. Which foreign country will the family just happen to end up visiting this time? Which pop star will the family just happen to encounter while there? And what unsubtle bit of physical violence will Homer be subjected to en route? Contract leprosy, perhaps; get raped by a panda; or maybe get his head trapped between two halves of a lowering drawbridge?

This was change all right, but change as an excuse for idiocy. It was desperately disheartening for those who cherished and loved the show's early years. Watching Homer hold forth on the topless women he'd seen on holiday in Florida, or Marge accidentally getting breast implants, you wanted everything to be revealed as a huge wind-up, or a cunning satire on trashy TV. But there was no hidden agenda. What you saw was what you got: a base, repetitive, unfunny cartoon.

And now, off the back of such a catastrophic decline, the movie has arrived. Is it too much to hope that it will, despite everything, turn out to be confoundingly brilliant? The omens don't look good. The trailers have majored in physical violence, including Homer getting repeatedly battered by a wrecking ball. And the publicity machine has been grinding away, humiliating into submission anyone who dares doubt the staying power of a show that's clocked up 400 episodes.

One thing's for sure. It's not the momentous occasion it would have been had the film come out when first planned. It's too late for that now. Indeed, with Channel 4 burying new episodes of The Simpsons on Sunday afternoons, rather than showing them at peak-time on Friday nights as originally promised, it might be too late for the TV show as well.

The residents of Springfield, Vermont, may soon be ruing that giant pink doughnut.

The Simpsons Movie is out on July 27. Ian Jones writes for tv.cream.org and offthetelly.co.uk

· This article was amended on Monday July 16 2007. The article above mentioned Walt Whitman's collection of poems, Leaves of Grass, as Leaves and Grass. This has been corrected.