Best Example: The Office
Although TV has now resorted to casting established A-listers, usually stars are born on the tube first. Sitcoms especially find themselves with a lead eager to leave the homestead and cash in their newfound stardom on the big screen. That's good and well for them, but their departure usually leaves a gaping hole that can't be filled. Case and point: the final season of That '70s Show, which tried and failed at carrying on without Topher Grace and Ashton Kutcher.
The Office faced the exact same problem during the awfully unfunny eight season, which installed James Spader as the enigmatic Robert California in the wake of Steve Carell/Michael Scott's departure from Dunder Mifflin. It took a year, but the currently ongoing ninth season seems to have finally settled into a rhythm that trusts the supporting cast—which was always funny in its own right even when Carell was there—to steer the ship on its own without a forced foreign element.