Screwed communities are screwed not by few bad actors (which live everywhere), but by the attention that users gift to them for free. By normal people you live or work with. Forum literacy is the key, and it’s not built into everyone by default.
Imagine a forum for doctors to discuss new treatments and everyone can join in, soon it'd be r/AskDoc and of almost no value to actual doctors
It wouldn’t, if every doctor there knows that they have to react with “Sorry, this forum is not for this kind of advice. But you can try these reputable sites: … to ask this question”. And similarly to attempts of other users to answer right there. Or, if not sure, to just stay silent.
Iow, community gets ruined when it effectively stops being a community and turns into a no-one-concerned bazaar.