Earlier quoted context omitted.
I think you answered your own question, the familiarity for users (seo & content) which is "end-to-end no-code" is massive (especially once extended with user-facing plugins). And you're not even speaking about making the move to Gutenberg yet. Just too much value there.
what familiarity? every single plugin's dashboard/admin page looks completely different.
It's not to say there aren't some that say sod it and have their own UI library. But quite a few leverage wordpress admin styles and conventions (such as the wp_list_table).
But the quality is all over the place, much like with composer packages.