You're arguing that internet user interfaces become better at hosting debates over time.

No, I'm not. I'm saying the interfaces got better at certain features of UX, like dealing with spam and trolls. Usenet could be intrinsically better at debate (in the hypothetical universe where it had a restricted userbase and wasn't dying of spam and other issues).

eg. imagine a forum where all comments had to be accompanied by an argument map but the forum didn't have any way of banning/deleting accounts. I have little doubt that the debates would be of higher quality, since argument maps have been shown repeatedly to help, but would anyone use that forum for very long? I have much doubt.