What about Project Treble? That was supposed to define a clear boundary between the hardware-dependent and hardware-independent parts of Android so that one could be updated without touching the other. The goal was exactly that, to remove the dependency on SoC manufacturers for system updates. Yet somehow this dependency still exists? Meanwhile GSIs (generic system images) also exist and run on everything where you could unlock the bootloader.
Yes, however the update still needed to be pushed by the OEMs themselves, as Google didn't want to force their hand.
Naturally they rather sell new devices.
GSI came later as Google decided to make way for most components to updatable via Play Store.
ART was the last one to make it on Android 12, specially since they couldn't ignore Java evolution any longer, as Java libraries started to become unavailable on Android.
Project Treble doesn't go that far. It allows you to boot a mostly hardware-independent ("generic") AOSP, but it's still dependent on AOSP version, hardware architecture (32- vs. 64-bit) and other details such as the partition arrangement, which varies by device. Project Droidian is trying to make it feasible to run something close to a mainstream Linux userland (hence, not just AOSP) on top of Treble drivers, and it's not very easy.
