Treble fixes the problem of "manufacturer who didn't do any customization can't update because Qualcomm never updated their drivers & friends" by ensuring that the level that the Qualcomms, Nvidias, etc... build against is forwards-compatible. So small manufacturer can just take a new android source drop from google and plop it on top of the binaries they originally got from qualcomm or whoever and tada! done.
It doesn't help with the "OEM injected so much 'customization' that it's a disaster to move forward and will forever be abandonware"