That's something I hadn't considered! I'd thought about how you could use the AGPL as a scarlet letter: "I'm the only one that can profit from this", since as

That's something I hadn't considered! I'd thought about how you could use the AGPL as a scarlet letter: "I'm the only one that can profit from this", since as the author you still retain full copyright and can do whatever you want with closed modifications. tptacek appears to be using it this way, to keep other companies from selling it, even if he doesn't care much about selling it himself. But I hadn't realized that in such a situation, the author would be completely unable to take contributions from others without full copyright assignment!