In reply to mark k..

Custom tools will be able to update at their leisure (or not at all, if they so choose): if site owners are only using an app to publish content, it will continue to work as normal: posts that haven’t been touched by Gutenberg will continue to be processed and displayed exactly as they are now.

Sorry, but to me it sounds like you are contradicting yourself. It is impossible IMHO to claim on the one hand that gutenberg is the only editor that will be, and on the other that people will be able to keep using their tools the old fashioned way if they so choose. At the very least there should be a big notice before the first gutenberg post is published, or a post is converted to gutenberg, that this is a one way street.