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.