I run another forum completely unrelated to woodworking. It's fairly heavily visited, although not as busy as this forum - about 7500 Members / 3500 of which are active. It's been online since 2000, and the bulk of it's members are yonger folks ages 15-30, since it's a video game based site. Therefore, there's a lot of stupidity posted. But amazingly there's also a lot of really rational, thought out discussions that go on for pages, and sometimes the discussions turn into mayo/mustard threads, where everyone seems to lose their mind.
We have a much larger moderating staff than SMC, due in part because many of us don't have the time to sit devoted to the forum, and don't have time to watch it 24/7. In fact, lately i've visited it less than i even visit SMC.
One of our rules is that we never delete threads. Ever. We also don't delete posts. The reason behind this is we never want to forget what's been said before. Moderators don't need to be protectors of the public to shield the members from information, or non-information whatever the case may be. If a thread gets out of hand it gets closed. If a thread has been started for the clear purpose of advertising, or the like, it gets closed and moved to a forum called "The Trash Can".
If for nothing else, we hold on to everything for posterity. And to say "Look at what's happened to this topic before, if you want to discuss it again, don't make the same mistakes."
I've never seen a forum that deletes as many posts as this forum has, and for reasons that I don't barely see as "infractions" of the TOS. Just because a topic's mood goes south doesn't mean it negates all the information and miles of text within it. What's wrong with just closing topics?
If for nothing else, the old topic serves as a map to what path not to take, when the topic comes up again.
Just my opinion, but understand that it's an opinion from experience, and it's an experience with people who are supposed to be more immature than the population here.