kinda funny how you mention that black mage has an high skill flooring, generally BLM is fairly simple in concept, but the ceiling (being encounter specific) is incredibly high (hense the high amount of bad black mages)
i'm on controller and most jobs can fill the vast majority of 2 hot bars (16 buttons each so 32 slots free, i generally use an 3rd for stuff like sprint and other things that i don't need in most encounters)
on controller you can even enable to just double tap L2 or R2 (the triggers) to access an 2nd hotbar, so honestly their excuse is just bland and doesn't work.
hope this bad trent will die soon, since it'll ruin the game for a lot, even some casuals are complaining about how each job is the same.