In reply to Joen.

It is not my personal intention or even prerogative to force Gutenberg on anyone. If we are able to get Gutenberg into a state where it excites enough people about the potential that blocks bring (check this out), and to a point where a merger into core is likely, then there are a myriad of things to consider before that could happen. I don't know exactly what it might look like or what's feasible, but the one 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?