As I see it, project treble will improve the update process, but not completely resolve the endless forking. You will still have the "vendor implementation", which is different for every device. It will contain at least the kernel and drivers (kernel/userspace), for which you, as a user, depend on the manufacturer to keep it updated, and which will probably not be mainlined. This means, that after two years or so, when the support runs out, you will still have a device, that does not get updates anymore. Only for a smaller component of the operating system.

Also keep in mind, that this will work for newer Android O phones only. We already have more than a billion phones (the number could be much higher, I did not google it right now), which will never get that improvement. They can be saved from being electronic waste with projects like postmarketOS.