New film Saturday Night celebrates the legendary comedy show's origins – but last weekend's poor new season premiere confirmed it's on rockier ground than ever. What went wrong? Saturday Night Live has achieved legendary status – not a good thing for what is still meant to be a living, breathing comedy show. That is clear from the back-to-back premieres last weekend of a movie about the series' beginnings, and the new season of the show itself.

Jason Reitman's film, Saturday Night, a loving but flimsy fictional account of the 90 minutes leading up to the first episode in 1975, was released on Friday. The next night you could have zoomed ahead almost half a century to watch the tepid, half-hearted 50th season premiere of SNL. Both suggest how much the myth of the show has overtaken its reality.

SNL has earned its stature, of course. It transformed television comedy and has penetrated American culture. In 2004, Rachel Dratch started playing Debbie Downer, who saw the bleak side of everything.

The term "Debbie Downer" is now simply shorthand for a wet blanket. And even a short list of the comic actors the show has produced is astonishing, from Bill Murray and Eddie Murphy through to Will Ferrell and Tina Fey, as well as the talk-show hosts Jimmy Fallon and Seth Meyers. Reitman's film goes back before all that, with the SNL writer Rosie Shuster (Rachel Sennott) trying to describe the show to a befuddled NBC network executive.

"It's postmodern, it's Warhol," she says of sketch.