I don't recall the exact wording, but the philosophy was to give them something good, but then quickly take it away again.
Buffy/Angel
Xander/Anya
Willow/Tara
Oh, Tara...
I loved having her on the show, but her passing also gave us one of the best arcs of the series with Dark Willow. She was kick ass and you could actually relate to her pain and even cheer her on - at first.
'Bored now.' I loved that they brought that line back!
Then she went all 'world endy,' and we got a 'villain' that Buffy couldn't beat. I've always loved that, that calm sweet Willow was the only baddie that Buffy couldn't get ahead of!
Exactly, it's just storytelling 101. Things get a lot worse before they get better, major loss eventually breeds new conviction & determination, character growth, etc. You had to get Willow over that addicted-to-magic hump eventually, it wouldn't be dramatic storytelling if she just came around to what Tara was saying willingly and that was that. We got a good balance, Willow making the effort, she and Tara ending up together, and then boom - giant cluster**** and Willow going all Cthulhu on everyone.
You don't build that type of power over like 3 seasons, narratively, and gradually include the slow-burn addiction story, only to never take it to its natural destination. As said earlier, Tara was basically over as a character by that point anyway - great as she was, what's she going to be doing in Season 7, Scoobies'-moral-compass aside? It was tough enough figuring out a place for Xander once **** hits the fan, it'd only be exacerbated having to do the same for Tara.
Sithborg, as for Anya, meh. She was never a "main", and she made it through to the final year, had a pretty huge in-depth arc over multiple seasons. Played her part, there's nothing sexist about offing her. Really no different to Book in Serenity, it's sacrificing someone for other people's character arcs.
Cordelia, sounds like Carpenter wanted out, and there'd been tension between she and Joss over season 4 with all the pregnancy-rewrite thing. Come season 5, she had a baby, she and Joss were at each other's throats the year before, I don't know if the Cordelia thing can even really be considered a "storytelling choice" or part of some Joss pattern. Charisma seemed to have wanted out.
I mean, they threw Doyle & Cordelia together at the end of season 1 then offed Doyle due to Quinn liking the tasty tasty heroin. But...penis, right? Doesn't fit the accusatory accepted narrative, guess that doesn't count.
Look, I get it, all of Joss's personal bull**** that's come to light. It's a major bummer, and personally I'd have a lot of trouble watching anything he puts out in future. But you look at his earlier career, the TV work, this "kills off ze wimminz" line just doesn't fly. If the female character deaths outnumber the male, it's not by much.