Back towards the end of last year, I did a few posts on Gutenberg for WordPress. Since then there's been a bit more clarity around what 2019 will bring for Gutenberg in WordPress.
A post on the Make WordPress blog explains how phase 2 will be about introducing Gutenberg to the Customizer:
This means that the sidebar, widgets, and menus will all be moved to Gutenberg.
Assurances that the Classic Editor will be supported until 31st December 2021 are largely worthless, as Gutenberg will be rolling out into Customizer in 2019 and this will fundamentally change WordPress.
Themes already need to be updated for Gutenberg as a post editor; entire new themes, or huge rewrites, will be required when the Customizer changes.