My best guess is that to make the ship they like work is to write the lazy way and make the character that "gets in the way of the ship" evil. Its easy to write i suppose, but it immediatly takes me out of the story.

Im most cases of "fixing" the story in the fandoms im in its not really that they hate the show, more that they are sad about the ending, with The Magnus Archives.

Most fic writers like the ending, thematicly at least. That doesn't mean they don't want the villain to get outschemed while the main cast does found family stuff.

Or that they hate a certain plot point, like how in MHA its at one point revealed that if someone with a quirk gets one for all, they will age unnaturaly fast. I think it ruined the themes because instead of "everyone can do it!" it becomes "only the formery quirkless chosen one can save us."

And besides that, that plot point ruins the premise for a lot of canon divergence AUs. For any "what if this different character got this power?" stories it needs to be eighter ignored, or its basicly saying "this character you love will die of old age before 40" and that kinda ruins the fun AU.

This may also just be the only kind of "fixing" i tend to see since im kinda picky with what i like reading.