AOSP has so few of the features a full phone needs today. Google has moved too much of the Phone OS into "Google Play Services". This is already the Extend phase of the classic "embrace, extend, extinguish". Given how the next most popular AOSP implementation, Amazon's Kindle Fire isn't even trying to compete in the phone space and involves an equally large company throwing nearly as much money into an "also ran" alternative to "Google Play Services", it seems easy enough to argue Android may even already be in the extinguish phase.

(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.)

There's not actually much in Play Services. The biggest losses are fused location providers and notification services which you would consider core to the OS. Maps are a loss, but these are very clearly Google branded.

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.