How can “Saturday Night Live” possibly replace (fill in the blank)?

How many times have we asked that question across nearly four decades?

“Impossible!” said some in 2006 when Tina Fey, Chris Parnell, Horatio Sanz and Rachel Dratch headed for the door, only to be followed two years later by her friend and “Weekend Update” co-host Amy Poehler.

But in their wake grew one of the most versatile, multi-threat casts in “SNL” history, one that firmly established its own “SNL” era. Kristen Wiig, Andy Samberg, Bill Hader and Overland Park’s Jason Sudeikis all became cast members in the 2005-2006 season, joining a group that already included Seth Meyers, Fred Armisen and Kenan Thompson.

At the time, “SNL” creator and executive producer Lorne Michaels pronounced them “the wave of the future” and Fey likened herself to a senior seeing “exciting freshmen” arrive.

But on Saturday’s season finale, Wiig got a musical sendoff as the popular and versatile cast member made her exit after seven years. She danced in turn with host Mick Jagger, cast members and executive producer Lorne Michaels to the tune of the Rolling Stones classic “She’s a Rainbow.” Then the ensemble sang another Stones hit, “Ruby Tuesday,” with its line, “still I’m gonna miss you.” Wiig appeared to be holding back tears.

Samberg and Sudeikis have been reported to be leaving as well, though Michaels has said any decision will wait until the summer. With a presidential election looming, an immediate exodus of all three is unlikely. Sudeikis plays both Republican candidate Mitt Romney and Vice President Joe Biden, and “SNL” has previously taken an all-hands-on-deck approach to election season shows.

Of course, the 2008 election season was a historic one for “SNL,” one that saw record ratings for the show as Fey returned – to much fanfare – to play Sarah Palin. This time around, no one is expecting Romney to choose a running mate that looks exactly like Andy Samberg.

A transition period, whether sooner or later, seems on the horizon. Perhaps more than any previous cast, this one has already expanded considerably from the show.

Wiig, of course, starred in and co-wrote the hit “Bridesmaids,” but even before that had notable roles in “Friends With Kids,” “Paul,” “Adventureland” and “Knocked Up,” among others. She has six films in some form of development, along with plenty of interest in a “Bridesmaids” sequel from her and her writing partner, Annie Mumolo.

Hader, who played Wiig’s husband in “Adventureland,” co-starred in “Superbad” and has numerous projects lined up, including a bit as Andy Warhol in Friday’s “Men in Black 3.” Samberg, who made the film “Hot Rod” with his Lonely Island cohorts, Jorma Taccone and Akiva Shaffer, co-stars with Adam Sandler in next month’s “That’s My Boy.” Sudeikis’ films have included “Horrible Bosses,” “A Good Old Fashioned Orgy,” “Going the Distance” and “Hall Pass.” He’ll also be in Jay Roach’s comedy “The Campaign.”

The typical path used to be to exit “SNL” with a film based on a popular character – as Will Forte did recently with the box-office disappointment “MacGruber.” But this cast has been as visible outside of “SNL” as it’s been on it. Armisen even managed to launch another sketch show at the same time: IFC’s “Portlandia.”

With a cast of half-a-dozen stars, there hasn’t always been a lot of airtime for younger cast members. Most avid viewers would like to see more of featured player Jay Pharoah, whose knack for impressions of Denzel Washington and Will Smith is so good that he deserves a chance to show more range. The same goes for the more consistently used Bobby Moynihan (who’s made his strongest impact on “Weekend Update” appearances, including as “Drunk Uncle” and as “Jersey Shore’s” Snooki) and Nasim Pedrad, most famous for her sharp Kim Kardashian impression.

But this season has made clear that if anyone is being groomed for a larger role, it’s Taran Killam. As a featured player, he’s become a regularly highlighted performer, including impressions of Brad Pitt, Michael Cera and Bravo’s Andy Cohen. More than the other of the younger cast members, he’s frequently gotten sketches into the show, like the Parisian parody “Les Jeunes de Paris” and “J-Pop America Fun Time,” a similar, Japanese spoof of American perspectives on foreigners.

Still, it’s been an uneven season for such a strong cast. The show has sometimes been overly reliant on predictable cable news frames for political sketches and leaned too heavily on recurring character sketches with so little variety as to seem like reruns.

But when “SNL” is firing on all cylinders, it can be as good as it’s ever been. This year, those moments have typically come when an alum has hosted: Maya Rudolph in February and Jimmy Fallon for the Christmas show.

Such occasions usually bring back other former cast members, as well. If anything, the “SNL” universe has grown larger, spread out across TV shows and myriad movies — making a kind of constant revolving door for “SNL” cast members, past and present.

In that way, “Saturday Night Live” has more in common with the mafia than any other TV show: No one ever really leaves.