I don't understand why spammers can't be fully stopped via built-in methods that prevent say, mass emails to more than a certain number of independent contacts upon account creation, for the first month maybe of service.

Why give up on this point? There's nothing that says true anonymity has to lead to spam. Spammers have the limit that they have to spam to , presumably make money, since they go via the 1000 tries and only needing one hit to win. They have a weakness. users and activists dont have this weakness really.