In reply to mark k..

Nobody is denying that some tools will potentially need to update how they'll work, particularly if they're tied very tightly to the markup that's stored in post_content. This will mostly affect generic tools, like the mobile apps, which are often used in conjunction with the wp-admin interface.

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.