There is no way they can ever balance the game by doing this. Level Sync and Daily Roulettes are not meant to benefit high level players first, they are first and foremost systems that are meant to populate lower level content to help new players not have long queues and feel engaged. That being said, to incentivize high level players to sync down and do old content, they reward you with gil, tomes, and materia (among other things like quests that give relic items e.g. Alexandrites/Singing Clusters).

A Lancer doesn't learn any AoE until Level 40; Doom Spike. That means a 80DRG syncing down has at their disposal, Doom Spike, Sonic Thrust, Coerthan Torment, Geirskogul, Nastrond, Dragonfire Dive, Stardiver. 7 forms of AoE damage while the level 39 lancer has 0. Even if they nerfed the damage down to 10 potency, it's infinitely more AoE damage than the low level player does. Look at healers, Benediction and Lustrate are oGCD. Low level healers have no oGCD yet. There's just no way to balance it without higher level players having a definitive edge in low level content. Moreover, high level players already have an edge, they are sync'd down to the max level for the content with max substats from melds, all their cross role slots available, and their job stone's bonus stats.