you never know. david caruso left NYPDBlue and it still survived.
Many shows survive departures of main characters. Most famously, people were concerned about Shelley Long's departure from "Cheers", which actually revitalized the show. Next season, Noah Wyle will be the only original cast member who's been on all seasons of "ER". "M*A*S*H" and "Charlie's Angels" also went through various cast revisions, to varied degrees of success.
The main difference, though, is that in none of these cases was the show actually named after the departing character.
But historically, this has happened to a title character in a series. After her dispute with the show's producers, Valerie Harper was fired from her show "Valerie", which was a family sitcom where she played the mother. So the following season, they replaced her with Sandy Duncan and the show was retitled "Valerie's Family". That was a transition title, and the year after that, the show's title was again changed to "The Hogan Family."
Getting back on-topic, I would never watch a "Dawn, The Teenage Slayer" spin-off. But if Eliza Dushku's movie career takes a nosedive in the next 10 months... count me THERE for "You Gotta Have Faith".