Yeah, I figured that there might be more power to killfiles than I knew about. Still, even if you block the cranks, all those non-killfile-using good people continue to pointlessly argue with the crank, and that poisons the group spirit and it drives people away. As a member of the community, you eventually suffer from the crank’s activity even if you don’t see his posts directly.

That happens, but it doesn't always happen. Some groups persevere while others succumb to the abuse. Usenet is still so huge that it's really not fair to write it off completely.

Usenet, much like SMTP (but arguably unlike XMPP) built enough momentum before the abuse exploded that there's no reason it can't continue in perpetuity, even if individual groups might come and go. But unlike SMTP it's much more at risk of collapse if good people walk away unnecessarily.