http://www.huffingtonpost.com/2013/03/15/tina-fey-admission-30-rock-muppets-snl_n_2878845.html

She tells The Huffington Post: "I feel like 'SNL' is so defined by Lorne's taste and his sensibility. That's why any time people have tried to imitate the format and make their own version of it, you'll notice it never really quite happens. And I think that's because he is the center of that show, and I think it should just — when he wants to stop, it should just stop."

NBC owns the show and if it's doing well when/if Michaels decides to retire, they will continue producing it. They didn't stop in 1980, when Michaels left for five years, so I doubt they'd stop now.

in 1980 NBC didn't have a clue what to do to replace Lorne, if he at least gives them some notice they could potentially have time to make a better decision who to replace him with

The 1980-81 season was a disaster when Jean Doumanian was producer. The pinnacle was when Charles Rocket said the f word on live TV. But the lone bright spot was the debut of Eddie Murphy as a featured player. He would later become one of the first breakout stars of the 80s.

in 1980 NBC didn't have a clue what to do to replace Lorne, if he at least gives them some notice they could potentially have time to make a better decision who to replace him with

I don't know if they'd have a clue on how to replace Lorne now, even with advance notice.

They need someone who's going to take a new approach and really reinvent the show.