I'm one of the few who is really of the opinion that the characters and franchise as a whole absolutely should continue once the creators/actors/producers begin to pass away or lost interest. I really do believe that Buffy the Vampire Slayer and it's spinoffs have the potential to be a truly enduring franchise, in the same vein (if not the scope and size) of Star Wars, Indiana Jones, Superman, Batman and Spiderman.
And really, why shouldn't it? It's a great series, with tremendous characters. All of the original Superman and Batman creators are dead, and both of those franchises are as successful and as big as they've ever been. If Christopher Nolan or Christian Bale said tomorrow that they have zero interest in continuing the Batman line, I would have no problem with new minds taking it over, even though that series consists of some of my favorite movies ever. Likewise, if Bale passed away tomorrow, I'd be sad and upset, but I wouldn't want the Batman character to be retired from film and television.
I'm an avid comic book reader, and three of my favorite comic runs ever are Green Lantern by Geoff Johns, Captain America by Ed Brubaker and Avengers by Brian Bendis. Since I've been reading, I've never known another author on any of those titles. In the last four months, they've all finished up their epic runs on those titles, moved on to new things and been replaced by fresh writers. I'm not necessarily excited about the prospect, but I love the characters enough to at the very least give the new titles a try.