Tim Walker: 'I still love Entourage – the way you love an ageing pet who's blind and lame'

When I first fell for Entourage, HBO's "dramedy" about movie star Vincent Chase and his friends enjoying the Hollywood high life, it wasn't Vince's charismatic super-agent Ari Gold that turned my head; it was Johnny "Drama" Chase, his older, less successful brother. Played to perfection by Matt Dillon's younger, less successful brother Kevin, Drama is an infuriating mixture of aggression, insecurity and extreme loyalty, equal parts loveable and hateful.

A dreadful actor with dreadful luck to match, Drama was fired from Melrose Place (a real programme), constructively dismissed from Five Towns (a fictional programme), and supposedly appeared as Joey in an unaired pilot of Friends, only to be replaced by Matt LeBlanc. But this week, after years of struggle, he learned that he finally has a holding deal with a TV network, to develop a new show with him as its star.

Entourage, meanwhile, has reached the end of its sixth season. I still love it, but I love it the way you love an ageing pet who's blind and lame and unable to defecate without a vigorous tummy rub. I love it the way I love Drama: I'll happily keep watching him audition for his spot in my weekly viewing schedule, but in my heart I know he's too old for this game.

I hardly need list the many successful series that outstayed their welcome, though Friends itself is the most glaring example of an over-ripened classic, now remembered as much for its interminably slow fade as for its superb early episodes. Entourage, too, was great for the first few seasons – brilliant, unlike other shows, because it was so entirely undramatic: four friends eat breakfast, drink cocktails, engage in casual sex with a string of beautiful women and casual conversation with an embarrassment of celebrity cameos, all the while charming our pants off with their repartee. The closest they come to jeopardy is wondering whether Vince's next blockbuster paycheck will cover the rent on their mansion in the hills. It's Sex & the City for dudes.

Then, around season four, the show's creators decided to go "dark", letting Vince (Adrian Grenier) and co make some bad decisions and some worse movies. The new plotlines required Grenier to be a better actor than Vince, and he wasn't really up to it – otherwise Marty Scorsese really would have offered him a role to rescue him from his career rut, as he did Vince. Now, thanks to his performance as Nick Carraway in Scorsese's modern re-imagining of The Great Gatsby, Vince is back atop the A-list, and Entourage is just doing victory laps. What could they possibly be planning that's new for a seventh season, now that Ari has his merger, Lloyd has his office, Drama has his holding deal, Eric has a fiancée and Turtle has UCLA?

One show that managed to retain its reputation (despite a finale that divided opinion) was Seinfeld, which has also found a way to make a comeback without overly endangering its legacy. Larry David, Seinfeld co-creator and the star of Curb Your Enthusiasm – in which he plays a version of himself – has reassembled Seinfeld's cast for a storyline in Curb, wherein the semi-fictional "Larry" persuades a reluctant "Jerry" and friends to make a reunion episode. If they pull this post-post-modern trick off (and their records suggest they can) it will enrich both shows.

Entourage, like Curb, has always delighted in the interplay between real celebrities and their satirical self-portraits. In just the last few episodes, Matt Damon, Bono, David Schwimmer, Aaron Sorkin and Zac Efron (among others) have appeared as themselves. Malcolm McDowell, moreover, plays Ari's former mentor and arch-rival Terrence McQuewick, who is married to Melinda Clarke of The OC, who plays herself. Thankfully, keeping up with which famous actors are playing themselves, and which actors are playing characters, is by far the most demanding intellectual challenge posed by the programme.

With so much playful, self-referential, meta-meta-television out there, Entourage could come gracefully to an official end and live on through cameos in other shows. Maybe Adrian Grenier, (playing either himself or Vince) could appear in Curb, as Elaine's toyboy on Seinfeld. And what's to stop Ari invading 24 to solve a hostage crisis armed with nothing but his Blackberry? Or Johnny Drama waking up in Flashforward, to find that in six months' time his holding deal will have fallen through, and he'll be back in the audition room all over again?