Ultimately defederating bad actors and defederating "good" actors who fail to moderate their own users is necessary.

Agreed, and this is what makes the Fediverse so good. It would be annoying to lose your instance, true, but you just move to another or roll your own. Additionally, let's say they start spamming Mastodon from mastodon.social; their messages would go to the Global channel, but if I only ever read Local or Subscriptions, I'll never see their spam.

The Fediverse and ActivityPub will continue to evolve, but unlike SMTP, they were created after the internet became adversarial. This author isn't the first to try to fearmonger over the future of AP, and they won't be the last.