It's primarily the chipset vendor (i.e. Qualcomm, Nvidia, etc.) at fault for lack of updates, not the OEMs. Treble removes one more excuse for OEMs to hide behind. (Not to mention the infrastructure is now in place for Google to push device-agnostic system images directly.)
I don't belive it will change anything, unless Google actually forces them.
My brand new 2017 LG stuck on Android 6.0 kind of proves the point.
For now, yes, but it opens up the possibility of changing that down the line or for users to manually perform those updates.