A CMS is primarily concerned with content modeling, workflow and collaboration. CMS are very complex systems and in this context, the content editor is only a small part of the whole. In any case I agree, I definitely prefer "blocks" editors, they are now a pretty consolidated solution.
As for the headless architecture, I would like of being able to choose what type of format to have in response to a query. In some cases I would like a representation of the content in JSON, in others an RSS feed, in other cases it might be useful for me to have a rendered and optimized version of the content in HTML or AMP that can be cached until the content changes.
In any case I will keep the public front-end agnostic preferring a very simple and SEO friendly approach. I like the idea of using Custom Elements, Fetch and History API for interactive elements and to make the pages navigation experience more pleasant. If I were to use a design system or a library to build reusable components, perhaps I would use them to develop the most complex part, the CMS application.
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