It's not the internet, it's the people on it and the people trying to control it for money or power.
The best example I've found of a self-regulating forum on the internet was one aimed at technical discussions of (mostly) computers, protocols, and so on. There was nothing preventing anyone from posting to it... except that if you didn't do it in a very specific way, which was not listed anywhere, your posts never propagated and no-one else ever saw them. And this, too, was never mentioned anywhere.
If you wanted your posts seen, you had to know enough about the underlying messaging protocols to realize that most standard interfaces didn't let you muck around with the guts of the metadata, find a program which did allow it (usually as a very esoteric and barely-documented option), get hold of that, use it to dissect existing messages and find the hidden metadata that enabled propagation, copy that into the metadata of your own messages, and set your interface up to do that automatically.
No spam, no brigading, no mods necessary. Just a purely technical hurdle that anyone could pass if they had the kind of knowledge that the forum liked to see in its participants (and was all available publicly, if you went looking). And while, in theory, you could get as*holes who had the knowledge or were prepared to put in that kind of effort to break into the discussions, they were extremely rare and tended to be ignored - there were user-filtering options also available.
Elegant, set-and-forget, and it worked amazingly well to keep discussions on-topic and relevant for years, if not decades.
These days, unfortunately, most platforms are owned by private interests who want to attract the maximum number of easily-fleeced users to an advertising/propaganda platform.