Earlier quoted context omitted.
The Pinephone can suffer from the same issue where people maintaining the software included may decide they no longer want to maintain their software anymore. It's no different for this. Either someone needs to step up to mountain it or you replace it with something else. Wanting to get rid of the rest of Android just because dialer and messenger are no longer maintained is throwing the baby out with the bath water.
Pinephone has multiple compatible os-s, while most android phones don't have alternatives and become unusable if android drops support for them. Also, Google and the big OEM-s have the incentive to break compatibility to provide you with the newest gadget, which is not true for the pines. yet.
There is no "Android drops support for them". It's "Google drops support for them". Google maintains AOSP, but if they stopped doing that, probably AOSP would survive in some form. Also I am not convinced that companies like Samsung would be happy to go with a completely proprietary Google OS, so chances are that they would get more involved into AOSP.
Pinephone has nothing to do with the OS, it is a smartphone. You can install Linux mobile distros (like PostmarketOS) on many mainstream phones (like Samsung, etc), and you can install Android on your Pinephone. That is completely orthogonal.