The goal was to provide a stable ABI like non Linux OSes, with all the plus and minus it entails, not to upstream stuff.

Project Treble was only the begining, besides none of the PlayStore acceptance requirements forces the OEMs to update the drives.

Project Mainline followed Project Treble, where Android was further modularized.

https://source.android.com/docs/core/ota/modular-system

Followed by GSI (Generic System Images), where AOSP can be plugged alongside modular component packages, thus allowing OEMs a more Lego like experience to creating custom Android builds

https://source.android.com/docs/setup/create/gsi