In the end it's just a way to make OEM's lives easier while developing updates. It will still require them to actually make updates for devices. Despite Project Treble they will still not have an economic incentive to update devices. And that is the core issue here, really.
My expectation is that Project Treble will help speeding up Android updates - they should come to devices sooner than they are released now, because Treble shortens the development process of updates. I don't expect devices to be updated any longer than they are now, or receive much more updates. OEMs will remain economically unpunished for not updating and likely will just pocket the money saved in the development process.
The only way to solve this problem is to build a "Windows Update for Android" whereby system components are updated in a modular and OEM independent way. The fundamental issue in Android's update problem is that OEMs are fully responsible for device updates and not motivated to do this job well, and Project Treble does not signficantly change this [1].
Don't get me wrong, Project Treble is a great move for Android. I however don't think it will make a huge difference in the Android update story.
[1]: EDIT: it's worth pointing out that Project Treble looks like a necessary move to make a "Windows Update for Android" happen though. Let's see if that becomes a reality.