What's the go with good shows get cancelled? All shows end at some point, its just that some end prematurely.
Thing is, why do they end. Generally speaking, it isn't because of the lead actor. Take Tru Calling for instance, that show seemed to be designed to be as it was in season 1. Then in season 2 the shows producers, directors, scriptwriters brought in a character that wasn't well planned, was played by a has-been. His character changed the shape of the show, essentially it was another show, and it failed.
The show could have continued as it was in season 1. The reason why the show failed was because of bad management.
And Dollhouse was like so many previous shows with that story line, they had been successful, so Dollhouse had a lot to live up to. But it didn't
Joss does seem to have success when he 'invents' a new character, and the show centres around that character.
Buffy was a new character, the show centred of her, the show did good,
Angel sort of did good, but characters from Buffy kept being used, and the show ended. It began to seem as though the show couldn't survive without using the ratings of those other characters.
Dollhouse, the show was centred on the business, had it been centred on the main character, well.
Firefly, the whole show seemed based on the theme of space cowboys of the wild west, that may have been ok in America but not so much in the rest of the world. Also the story lines needed to be powerful stories, that were accepted worldwide.
So don't be to ready to blame the actors if the bosses get it wrong.