In fact, I talked about not reinventing the wheel. It should actually work the same, have the same roots, base, database structure, architecture. All cool. What I mean is…
If to integrate X functionality then a bunch of WP 4.9 plugins have to stop working because of this or that, then well… that’s how it goes, just keep walking.
Anything else will just be wasting time on creating weird tools like (detector for when plugins stop supporting WP 4.9) or minor UI tweaks like “center the logo here”, “change the font there”, “make this bigger”… but I’m sorry to say that is nothing close to “WordPress but better” . It’s actually quite the opposite. 
Examples of cool stuff that would make CP be something really cool. And none of those topics were started by myself, it’s people who want things to happen!
- GraphQL API
- ACF in core with off switch
- Easily rename “wp-admin”
- Better DB maintenance
- Use TinyMCE v5
- Improved media library
- Move comments system to switchable core plugin
- Disable XML-RPC by default (move to core plugin)
- Move to Vanilla JS
- Allow renaming files in media library
Also, none of this is reinventing the wheel and moving “away” from WordPress. It’s actually WordPress (but better) as you mentioned. The thing is that implementing some of those will break compatibility with themes/plugins. But as long as there are solid alternatives tailored for CP, like Classic Commerce, Classic SEO, etc… what’s the problem?
The only problem is: some people want to use WP 4.9 plugins… well, let them use WP 4.9 or whatever. Let’s focus on those who are on the other side, which is actually the only side that makes sense from a practical point of view.