A lot of people don't like to compare Gutenberg to Page Builders. I feel like once you start allowing the user to define layouts you've moved away from simple editing and stepped into realm of page building. You're no longer editing simple content in a word document like the Classic Editor but building layouts on a white screen that may not necessarily look the same when you publish that page to whatever theme.

Part of the beauty of Page Builders is you get the front-end experience while building your layouts and adding in your content. They may as well start to embracing that type of experience and start competing with actual Page Builder plugins.