I was thinking the same thing, Treble sound like the perfect solution for the kind of problem this project aims to fix, but I realised that the scope of this project is different. There's no way they can back-port Treble to older devices, and there's no pressure from consumers or manufacturers to do so. Project Treble aims to fix the fragmentation problem, but it does from this point on, older devices are not included.

There's also the fact that Android will keep on rolling, new features will be added and manufacturers will keep on using them as they see fit, so Treble can help users with a security update or two while that version of Android is supported, even if the manufacturer stops supporting their "vendor layer", but what will happen when Google drops support of Android O?