Now that making apps is easy, and it's the apps and eco system etc that makes an OS take off and adopt, starting a new OS for phone like devices shouldn't be as big of a problem as it used to be.
Given fair phone exists, it should now be able to get all kind of apps anyone wants.
Ever tried using Linux on real mobile hardware? Phosh and updates via Flatpak are unbearably slow. I have yet to see a way to try Plasma Mobile, etc or have much of a way to customize the filesystem and lower level services to make things better.
The problem isn't missing app functionality that anyone can build. It's access to proprietary third party services that are too useful to go without. And they're not going to let you build your app to access their services.