There was always this curve where a forum would be fun because it was a small group of likeminded individuals. And as it got larger, more trolls came in, more newbies with different interests, and others that might not have shared the same cultural mentality that started that forum. And the forum would begin to shed its original members who draw the crowd. Then what’s left is people who have less interest in the community spirit. And that forum starts to shrink again as more people become disinterested and more infighting happens.
Good moderation can help but nonetheless I’ve seen this happen across dozens of forums. In IRC channels, newsgroups and so on too. So it’s not even a phenomena specific to HTTP message boards. Some communities even have a terms for it like “the eternal September”.
To be clear, this is a separate problem to the addictive design of modern platforms. That’s a whole other level negativity. But people seem to forget that the internet could be a cesspit even before Zuckerberg poisoned the well.