If Gutenberg was better than third party page builders, it would make onboarding new clients easier.
Fewer competing tools and integrations to worry about.
This is not the case- yet.

Perhaps page builders will become block editors in the future or just an interface for creating blocks.

Gutenberg is not even Wysiwyg yet. Page Builders are still better user experience.
WordPress needs to focus on making better options than third parties, rather than replacing options just for the sake of it- or to push their own roadmap ahead of the ecosystem.
If Gutenberg is better than a Page Builder, more people will use it naturally.