I expect to burn a few karma points on this, and be ignored, but all evidence I have says that this is human nature at work, and that at scale it cannot be prevented.

I've seen moderators of small communities hold off the effect by setting a good example, using a deft touch, and not letting the community get too big.

My suspicion is that once the number of commenters gets much past Dunbar's number [1], the universal fall into "default jerk" happens. Once that's taken place, you're stuck in an Eternal September.

Communities in the real world don't scale well. I'm not sure why programmers expect them to on the Internet, with its well-known capacity for dehumanization.

1: https://en.m.wikipedia.org/wiki/Dunbar's_number

I think there's something to this. Not sure if it's Dunbar's Number specifically, but something. It seems like there's a pattern where a community is nice and small, with people joining at a trickle. Things change when a jerk joins and starts doing jerk stuff. This tends to drive away the nice and helpful people, attract more jerks, and pushes people still there to be more jerk-ish themselves. Sometimes deft management can prevent it with some well-timed bannings, but I think this gets increasingly difficult as the community gets larger, and may be effectively impossible in internet-scale communities.

You're describing the broken windows theory [0], it's not related to Dunbar's number.

[0] https://www.britannica.com/topic/broken-windows-theory

Yeah, I'm not sure it's Dunbar's number precisely - it was just what came to mind.