Over almost four decades, Saturday Night Live has displayed a Madonna-like gift for reinvention, defying critics who, every five years or so, question the show’s relevance in a rapidly changing culture. The likes of Chevy Chase, Will Ferrell and Amy Poehler have come and gone, but SNL has endured.

This year, SNL faces one of the most significant talent exoduses in its history — perennial stars Kristen Wiig and Andy Samberg are moving on, Jason Sudeikis is likely to follow, and after 11 years head writer and “Weekend Update” anchor Seth Meyers appears ready for another challenge — though he’s denying rumors he’s replacing Regis Philbin at Live! With Kelly.

Collectively, these exits could be the greatest challenge the comedy institution has tackled since the high-profile departures of Phil Hartman, Adam Sandler and Mike Myers in the mid-1990s. Growing pains are certainly nothing new for SNL or its creator, Lorne Michaels, but this latest changing of the guard comes at a particularly awkward time: NBC, which has languished near the bottom of the network ratings since 2004, has more than enough problems without worrying about the status of SNL.

“ SNL has to perform at a certain level,” says Brian Steinberg, television editor at Advertising Age. “NBC can’t afford to have any cracks in the pillar.”

Both Michaels and NBC Entertainment Chairman Robert Greenblatt declined to comment.

According to NBC spokesman Tom Bierbaum, the show has continued to hold its own. SNL ratings have held steady at an average of 7 million viewers since 2004, except during the abbreviated 2007-08 season. This year, the show averaged about 7.1 million viewers, down just a hair from 7.2 million last season. To put these numbers in perspective, that’s as many as watch “The Biggest Loser,” and double the usual audience for The Tonight Show With Jay Leno.

Not bad for a show that’s about to turn 37.

SNL also performs well among the coveted 18-49 demographic — especially when compared to the geriatric hinterland that is Saturday night television — and in particular young males. This is a draw for deep-pocketed advertisers, like beer companies and film studios, Steinberg says.

Still, with a large ensemble cast, an even bigger crew, and wardrobe and costume expenditures that would make J.Lo blanch, SNL is by no means a low-budget production. According to a source close to the show, it costs NBC about $3 million to make a single episode, a budget comparable to that of an hourlong network drama.

Though it’s highly unlikely SNL would lower the boom on the show while Michaels is still involved, budget cuts are not unprecedented. In 2006, Michaels axed five cast members under pressure from the network to cut costs.

Whatever the investment, it’s worth it, according to TV analyst Shari Anne Brill: “There are people who came into SNL who don’t watch anything else on NBC. They need to remind people that they have shows on the rest of the week between 8 and 11.”

James Andrew Miller, co-author of Live From New York: An Uncensored History of Saturday Night Live, acknowledges the show is at a crossroads. In particular, he calls Wiig’s farewell “one of the most emotional departures in the history of the show.” He cites her ability to portray a wide range of characters — not just the wacky ones, like Target Lady.