Google has published some statistics about the effects of Project Treble on Android updates.
In late July, 2018, just before Android 9 Pie was launched in AOSP, Android 8.0 (Oreo) accounted for 8.9% of the ecosystem. By comparison, in late August 2019, just before we launched Android 10, Android 9 (Pie) accounted for 22.6% of the ecosystem. This makes it the largest fraction of the ecosystem, and shows that Project Treble has had a positive effect on updatability.
That’s definitely good news, but Google still has a long way to go.