The problem is that right now, in the common case, the browser/OS vendors can't black-list individual CAs. Their ability to do so with DigiNotar is exceptionally rare, and would not be possible most of the time.
Trust agility ensures that clients have the ability to make these trust decisions easily. A client does not necessarily have to be a user, it could still be the browser/OS vendors. For details on how Convergence works, in order to answer your question of how it prevents 1/4th of the SSL internet from going down when Comodo gets hacked, the best reference is (unfortunately) still the presentation: http://www.youtube.com/watch?v=Z7Wl2FW2TcA
The presentation cleared things up marvelously. It may be worth adding the presentation to the convergence.io details page, even if it was just a clip of the last few minutes where you talk about notaries. Once you went through that, everything cleared up.
Trust agility ensures that clients have the ability to make these trust decisions easily. A client does not necessarily have to be a user, it could still be the browser/OS vendors. For details on how Convergence works, in order to answer your question of how it prevents 1/4th of the SSL internet from going down when Comodo gets hacked, the best reference is (unfortunately) still the presentation: http://www.youtube.com/watch?v=Z7Wl2FW2TcA