"What makes you think there wouldn't in practice be the same centralization that we see today -- one or two paid services that handle moderation for the entire Internet because only they have the lawyers necessary to do so."

One important difference is that the networking effect is not as big as in case of FB et al.

But yes, the general trend in maturing markets is toward some kind of oligopoly. It is, I believe, time to reactivate antitrust laws, we are well beyond the point where they should have been used.