It's hard to be a fan of The Simpsons in 2014. We already know that the show isn't even a shadow of the all-powerful cultural colossus it was back in the 1990s, but now – in the winter of its years – it hurts to see it lurch from attention-seeking gimmick to attention-seeking gimmick without a trace of shame. First it piggybacked on Game of Thrones by announcing a character's death as the Yellow Wedding. Then it piggybacked on the success of The Lego Movie by making an episode entirely from Lego.

But now? Now a brave new depth has been plumbed. There's going to be a Simpsons/Family Guy crossover episode.

The Simpsons has featured characters from other animations before. The 1995 episode A Star is Burns brought in Jay Sherman from The Critic, for example. And while that episode had its moments – notably "I was saying Boo-Urns" – it tore the production team apart, with Matt Groening and James L Brooks publicly airing their dirty laundry about whether or not it compromised the integrity of the Simpsons' universe.

But that was almost 20 years ago, and quality control has dipped enough to render these concerns null. The Griffins are coming to Springfield and there's nothing we can do about it.

What makes this especially hard to swallow is that Family Guy is, at heart, a crap Simpsons.

Watching an episode of Family Guy is like watching an episode of The Simpsons where all the heart and sincerity have been boiled away, leaving a twitching mess of arbitrary cultural references. Peter Griffin is Homer Simpson's id run amok. Lois is a terrifying Mommie Dearest caricature of Marge. Stewie is one joke stretched out to abysmal lengths; basically what Bart would have been if The Simpsons had allowed him to remain a catchphrase-spouting annoyance. The Simpsons has a vast fleet of supporting characters who, even to this day, are richly drawn enough to shoulder entire episodes. Family Guy has a man who says "giggity" 12 times an episode.

It's impossible to work out why The Simpsons would debase itself by allowing this to happen. It's like turning up at your favourite bar – the bar you've grown up in, which is full of interesting, sophisticated people – to discover it's been overrun by a stag party of bellowing dimwits who chest-bump each other and try to pour beer up each others' bottoms. Look, the creator of The Simpsons went on to create Futurama; a show full of brazenly intelligent maths jokes. The creator of Family Guy went on to create a million shows that are exactly like Family Guy and the world's worst tedious swing album vanity project.

According to the handful of vague plot details released so far, the crossover episode will show Bart teaching Stewie to skateboard, Homer and Peter getting drunk together and everyone who ever loved The Simpsons walking to their bathroom mirror during the commercials to stare hard at their own reflection while repeating positive affirmations about how it will all be over soon.

Fox's entertainment chairman has declared that "generations of fans will be talking about this one". It's too early to call, but there's a good bet that what they'll be saying is this: Worst. Episode. Ever.