We have enough copycats of Cheers now to know that the successful ones can run pretty much indefinitely. Friends shook things up by shifting attention to Monica and Chandler, and by doing stunt casting. The Big Bang Theory just married the guys off and turned into a domestic sitcom. I think that Cheers could have just married Sam and Diane off and had Diane raise kids at home while Sam bounced back and forth between home and work. Same basic formula as worked for a lot of popular shows, including Mary Tyler Moore, Home Improvement, and Frasier.