It wouldn't be appropriate on the forums I run but a few years ago I installed SMF on a domain with no defined niche and no spam prevention as an experiment. I let it run without management or moderation for a couple of years and during those two years it attracted thousands of members (or bots) and over a couple of million posts.
I saw something similar. 3 times, in fact.
I was a part of a large 2.5 Million post board, it had a what's called a "madhouse." Spam, chitchatting, trolling. Every thing you can throw at it. It was a private forum, at the peak it was chaos. It was actually inspired by two forums that had this model, but could never reach scale like this board - with 25 thousand members by the time it was sold. Everyone stopped going because of the drama, followed by Facebook, followed by crypto, and then the owner quietly shut down the forum. Talk about thousands down the drain. Idiots.
2 other situations were similar, because they are actually part of the same community. Its literally the same exact story of NeoGAF admins/mods splintered off to make ResetEra thinking they'll "do better." Turns out - THAT board also got splintered off - famiboards. And, AND... ResetEra GOT SOLD... for 5 Million. (I think..?)
The first board was sort of splintered off from IGN, but it was not a mass migration off ign, like NeoGAF was. It was its own board. A small, but thriving community that is only accessible if you registered. By the time it was shut down, a small section of that community splintered off to make a new community. It died because of drama. It had 1 million posts. The owner hated the drama.
What happened to that small community? It got killed by Kenny. (South Park reference) Once drama spiraled, the board was shut down. I forgot why, but I think its because the owner doesn't know how to keep the site running without ads or funding. When they tried to revive the site, they uploaded the database to upload website. I downloaded it, restored it. Put it on my site, emailed members, they got.. MAD.
I did it; I killed it. I got pissed off that's why.