There have been plenty of instances where someone just turns off a subreddit, now nobody gets to use it, have a nice day. Let alone the more subtle problem of a toxic person who should not be moderating a sub in the first place.
I think this hits on the underlying problem I see with Reddit and similar forums, though. Look at the evolution of subs like /r/LateStageCapitalism or /r/FatPeopleHate or /r/Incels or T_D or TRP or Flat Earth or Broneyism or whatever. I don't think in all cases the communities started out as extreme as they eventually became.
I'm not defending their original charters by any means, I just think there's some kind of sociological reality or formula these subs are tapping into that allows them to purposefully moderate/evolve into echo chambers and bring a community of followers along with them, to a point where the community even starts to self-moderate to the extreme -- but they don't just start out that way.
It's almost like you can take some ridiculous idea or some interesting but archaic belief system, build some interest in it using humor or shock value, then once you have an audience with critical mass slowly turn it into a cult without anyone noticing, like the boiling frog analogy (hmm, the irony of that comparison just now struck me). I almost want to try this myself with something absurd just to prove the theory.
And I'm sure this isn't a new concept in sociology or anthropology and there are people researching how it works at Internet scale. Can anyone point me to what it's called?