In reply to Joen.

Matías suggested as an option — “If we detect a meta-box is registered we can fallback to the old interface, nothing changes” — seems like it would be pretty transparent no?

Agreed. This is probably the least intrusive path forward for metaboxes in particular. If I had to guess, the vast majority of WordPress sites have at least one plugin registering a metabox. Would merging Gutenberg into core be acceptable if only a small percentage of users even see it? I understand the idea is plugins will update as time goes on, but out of the gate it would lead to all kinds of confusion.