Yeah. Phpbb was a huge deal in the early 2000s. Then reddit came along with subreddits and Facebook groups took over. All stuff now happens under the curated moderation of the corporation that owns whatever hobby/interest.

You're omitting a major driver of the exodus from private forums to walled gardens. It's the same driver of many other problems: assholes.

Spammers killed many a forum because it became a full time effort by admins and moderators to remove them. Automated tools, especially early tools, had limited effectiveness. People running a site as a hobby don't want to spend their days removing spam comments.

Assholes also (for many reasons) will DDoS forums. This is another administrative headache for site owners if not a financial headache.

The same driving force saw blogs remove comments for centralized solutions like discus or abandon self-hosted blogs for Medium and the like.

While Facebook and reddit have plenty of spam issues the "owners" and mods of a group/subreddit are different from the site admins. There's a lot less financial risk for someone running a group there than their own server. They also let a wider swath of people run a group rather than only those that are interested in a niche topic, have the technical skills to effectively run a forums, and the money to pay for it all.