L4 runs on billions of devices but it's mobile modems, car infotainment, and stuff. There's also been innovation generally in the mobile and restricted space, no desktop OS can manage application lifetimes quite as well as Android can: Because it needs to because you want both multitasking and fit everything into limited RAM.
The problem is really the "rewrite everything" part because to make current OSs much better you'd need to work on the interface between programs and OS, and not just piecemeal stuff like wayland. It was possible on mobile because everything was new, anyway, but on desktop? Truth be told our best bet at new standard APIs is wasm.