The first time I "met" people who weren't involved in tech online was probably Friends Reunited (school friends) and then Facebook. Reddit, Twitter, Instagram, Snapchat and TikTok are all far more diverse than Usenet ever was (although I only ever use Twitter these days, so that might have changed).
I therefore can't accept your view that it had a dramatically diverse group of participants. It has an even less diverse group of users today, and it's worth remembering we should probably not talk about it exclusively in the past tense: it's still an active thing.
On the authority thing: the OP link suggests a public shared space has to be owned collectively, and therefore the policing model that Usenet lacked (other than a few good actors at the edge of the network like you suggest), will eventually be its downfall. We see this in unmoderated spaces all over the Internet today.
All public spaces are at risk of anarchy without some sort of policing, once populations reach a certain size. This is not a uniquely digital/online phenomenon.
I wonder if it's possible to create a protocol where policing is built in somehow, whilst retaining the public commons features that the OP desires.
