Applause for cleverly and diplomatic avoiding mention of Metaboxes and custom fields.
Nobody seems can and will answer this.

Many Metaboxes cannot be Gutenberg blocks. Never ever.

One step in right direction would be to stop discussing (on Github page) dual mode, theme support fot Gutenberg, and post type without Gutenberg (old TinyMce).

First, this is not possible. I do not have any accurate statistics, but I have never build an website where native "Post" post type did not have at least one non-native Metabox. If it was case I made native "Post" post type hidden anyway.
Second, seems you are on way to remove TinyMce from core.