In reply to Milan Petrovic.

One option would be to integrate it into core, but with an "add_theme_support" activation required. Default would be classic editor, while those that are ready can activate Gutenberg.

That meets the goals of releasing with 5.0, but also allows more time for it to be refined before making it the default editor with 5.1 (or another version).

Also helps prevent broken sites and doesn't require installing an additional plugin (which many users I've talked with don't even know exists).