There is still a lot unclear and a very contradicting message, reading comments in this thread.

On the one hand, Wordpress developers are reassured their website with custom post types and meta boxes will not break.

On the other hand, Matt is urging everyone to learn Javascript and React and to make sure to update their plugins and themes in 2017 ("Plugin authors should absolutely start updating their plugins to work with Gutenberg this year, 2017.")

I have about 70 client websites build on Wordpress, most using custom meta boxes. All very small businesses or non-profits. One of the main reasons I chose Wordpress is that changes almost never break anything. I can come across WP sites on very, very old versions, update them to the latest 4.8.1 and everything still works. Themes I build 8 years ago still work.

Who is going to explain to those clients their websites are going to break? Who is gong to explain to them they need to have their websites rebuild (that's what it comes down to)? Who is going to pay for the massive amount of work needed to rebuild meta boxes in Gutenberg blocks?

This is not hyperbole. Rebuilding those 70 websites will take me many months, at least. Most clients will not pay for that. But I also can't leave them with websites not being upgraded. And even if I somehow manage to rebuild everything, nothing has improved. For example, a client who needs to edit their "Project" or "Event" custom post type still fills in a couple of form fields.