I'm really interested in the point that you are making about 'there's no coming back' at the level of the community (i.e. ratio of 'those folks : total users) and the level of its members (i.e. deciding to leave) - do you think they both register with members/community in the same way, as either are happening? As in, other members in the HN community may not be aware if another member decides on 'not coming back' at any one point in time, whereas it could be the case that all members of some certain relation to the community (i.e. 10 years of membership or longer) knows when the HN community is not what it was once.

By contrast, is there a case to make for when 'an un-informed, un-thoughtful, un-pleasant person decides they're not going to post anymore'? If so, what would you say that tells us about an online community?

Someone wrote a post and called that "evaporative cooling." Edit: https://hn.algolia.com/?q=evaporative+cooling

Of course, Eternal September also applies and HN likely suffers from a low level case of it. If you aren't familiar, it's easily found on Wikipedia.