But Joss Whedon is a rodent of a man who still publicly holds a grudge against Donald Sutherland for the Buffy film, publicly fell out with James Marsters and sacked Charisma for getting pregnant. He doesn't like it when things don't go his way. The line about having to let Glenn go because of drug abuse didn't come out until after he had died (and could no longer speak for himself) putting the blame for an unpopular decision firmly on Glenn. But Nicky Brendon was abusing alcohol towards the end of Buffy, Eliza Dushku was on drugs ... he kept them around. I think he got rid of Doyle because he wanted to get rid of Doyle. The drug line, when it became convenient, was his cover.
I have to say that the more I read about Whedon the more I wonder about how utterly SMG's career stalled after Buffy. I remember in the first few years of the show reading about how Joss and most of the cast were great friends except for SMG who was a bit stand-offish with them outside of work. I remember the implication coming across that she thought she, as the star, was a bit too good for them and being a bit disappointed in her. Knowing what I do now about how Joss favoured people, especially women, he could manipulate and how down to earth and non-hollywood SMG genuinely seems to be, I think she saw through him and just kept their relationship professional.
Gellar is actually a very fine, very under-rated actress. There is a scene at the end of the S5 episode Intervention where Buffy comes to Spike pretending to be the BuffyBot. Throughout Buffy, SMG does a great job of keeping Buffy and the bot as utterly distinct from each other. But in this scene she actually does a fabulous job of being Buffy imitating the the bot. Her slight hesitation in her movements and her very subtly readable determination to push through even though she is uncomfortable comes through as she enters Spike's crypt. It's a blink and you'll miss it moment but it takes real acting chops to get all of that into a short walk. And then after Buffy as Joss' star rose, Gellar became more and more obscure. While those actors who were still playing Joss' games did pretty well for themselves.
I know there are plenty of talented actors who were so defined by their break-out role that it limited their career progression afterwards. But in recent years we've heard more and more about young women who have had their career sabotaged for not playing along with powerful men. And Charisma Carpenter certainly seems to have fallen foul of Whedon's power games. So I do have my suspicions that SMG's lack of success after Buffy may also be at least in part, for similar reasons.