A similar thing happened with Drupal when they pulled the guts out and rebuilt it on Symfony. A group forked it to Backdrop.
It has struggled to get much traction, but in some ways it doesn't need that much traction because they are not focused on adding features. It's for people who were happy with Drupal 7 and that's what they want.
The thing that worries me with projects like that is
- do they have the energy to keep up with security patches
- do they have the energy to stay compatible with third-party library changes.
For example, Classic WP still uses TinyMCE (I think) and JQuery. You can strip them both out, but these packages evolve, so you need at least a minimal community to keep things rolling.
More generally, I feel like the strategic decision for Drupal was to make it more friendly to people who were professional PHP developers and less friendly to everyone else. This was not a bad decision since they were watching the "everyone else" crowd switch in droves to WordPress no matter what, so catering to that crowd was just a losing proposition.
WordPress still seems to want to have it all - an easy on-ramp for the masses and more developer features and ever-increasing complexity. But because WP prizes backward-compatibility, as the complexity grows, the develop experience becomes more and more of a nightmare. Because of the fundamental architecture, it is really challenging on a WP site to use modern developer tools like Composer, which is pretty much required to safely run any complex PHP package these days.
I do not see WP ever being willing to break backwards compatibility in a serious way, so they are sort of stuck with an app that gets more and more complex, but is still incompatible with the tools that help you manage complex apps.