In reply to David Aslan French.

The "people are resistant to change, this is why they are complaining" is such a sweeping generalization.

As a developer interested in using Gutenberg, I believe that Gutenberg is good thing long term, however, it is still a long, long way away from being realistic to be entered into core.

I have personally engaged with the Gutenberg team with the odd question on how to do basic things for themes to play nicely and integrate with Gutenberg. And the answer has consistently been "Not possible at the moment/No one knows at this stage".

Essentially, it feels like Gutenberg is a closed plugin right now doing its own thing. To be ready for core, it needs to feel the exact opposite and be properly extendable in every way for themes and other plugins to hook in and extend it in every way just like WordPress is. And this goes well beyond the ability to adding custom blocks.