the constant barrage of negativity toward a character and pairing I didn't really have a problem with (and by extension, the actress/writers/show/showrunners)

I wonder if that's not part of the issue.

There's a lot of distress about the way the show ended, with the PTB focus on SGU (as sheafrotherdon mentions below) piled on top of the normal "Wah, my show is going away" reaction, and I wonder if Keller-hate isn't a way to identify as a fan of the show, by taking the position at the other end of the room from the writers who see the relationship as the reward they wanted.

So the overwhelming and pervasive Keller-hate that Astridv iss eeing isn't so much misogyny of slash as opposing (resenting?) (defying?) the PTB.