Unlike AOSP, SailfishOS has a largely closed source user interface and application layer. They still have their own UI and application layer so it's strange to portray Android using a different one than desktop Linux as a problem and a closed source alternative to it as a solution. SailfishOS drastically reduces privacy, security, usability, battery life and app compatibility compared to simply using AOSP. It's not possible to build a project like GrapheneOS on top when so much of the OS code is closed source rather than only many mainstream apps people want to use.