(ETA: See also Microsoft's many years of trying to build its own "Google Play Services" competitor. Eventually breaking and making use of Amazon's. Then giving up entirely again on a de-Googled alternative to running Android apps.)
Huawei provides HMS for example, a somewhat close feature wise set of APIs for their phones that are still on Android. They can even shim play services API, the same way microg does. If anything, what would be needed would be a common abstraction library with different backends to not depend directly on play services
The reason amazon and Microsoft gave up is because they had no commitment, and that operating these services is just pure loss.
Yes, the default apps in AOSP suck. Making a proper dialer is a two day job, so is a contacts app. Android's core APIs are good enough, and privileged permissions are only privileged by the manufacturer, and its IPC mechanisms are very well documented. Noone does it because it sucks, it's a thankless job and nobody's going to install your dialer. The very fact that each manufacturer has their own custom software is demonstration of how easy it is.
Building and maintainance cost are not linear, especially when you inherit legacy code. The AOSP codebase isn't great, is 4x bigger than the Linux Kernel, and full of "Ship now, patch later" mess.
But I agree that it is a significant endeavor. But the OSS community succeeded in similar projects before, and the current state of the Linux desktop makes me hopeful.