The 20% Balance was part of the entire snowball effect of buffing jobs from 3.0 to 3.4. Every time they buffed a job they overdid it.

AST actually got 2 buffs during Heavensward, one in 3.1 to make it a better healer and then again in 3.4 to kill WHM off.

They did the same with MCH (10% Hypercharge effectively killed having BLM in a meta group) and then buffed BRD because it was doing less damage than MCH. The whole thing snowballed to create a ridiculous situation where ranged were the top meta DPS and the two melee were basically support jobs for the party's damage output. The actual highest DPS jobs (MNK and BLM) were left out because they were "selfish dps".

I'm hoping SE has learned from this, since no doubt adjustments will need to be made during 4.0.