Gutenberg solves one huge problem that Custom Fields have, but at the same time, it negates 2 huge problems that Custom Fields had already solved... let me explain:

WordPress when used as an app. like a real estate site for example that uses Custom Fields, can have front end forms and parametric search pages, so an end user for example can find a house in Los Angeles, that has 4 bedrooms, and it is priced between $500,000 and $750,000. If this was done with Gutenberg today, we could not have a form on the front end for agents to list their properties, and the end users won't be able to make complex queries to find their perfect house to move into.

That said, as it is with Custom Fields, these kind of websites are not easy to scale as each field takes 2 rows in the table for each post. As an example if the app. is using 50 custom fields for each listing, and let's say that we have only 5,000 property listings, we would have a total of 5,000 rows for the listings (posts), and 2x50x5,000 = 500,000 rows for the custom fields for a grand total of 505,000 total database records (rows). Now if Gutenberg can solve the front end forms and parametric search issues and we can use only Gutenberg to create such sites, the database would only have 5,000 total records as oppose to 505,000 !!!

As long as these issues are not solved, Matt's dream of total market domination and making WP the operating system of the web is only a pipe dream and not a realistic goal at all. And my understand is that having front end editing or solving the parametric searches are not even in the radar of confronting this issues with Gutenberg, therefore WordPress will stay as a very good but an "incomplete" or not perfect solution to create large, real money making websites. Perfect for small, to medium sites, but never for large ones....