I really don't get why they just don't:
1) Check the expected dps of a job at X level, at each level a dungeon's unlocked at
2) Calculate the expected damage at each level cap (70, 80, 90, 100, etc) per job
3) Make a buff per job where it nerfs your damage appropriately based on how far you're jumping back level-wise
This would basically require one "base" reading per job per expansion, with minor adjustments besides. It wouldn't be sufficient for high-end content like savage/ult (which require tighter tuning), but wrt duty roulettes it'd be a nice way to keep balance somewhat in-line without having to remove player abilities.