Very well. If you think that casting a spell while moving once every 30 seconds, would generate enough dps to necessitate a potency nerf, then the cooldown can simply be raised. There are multiple moving parts in this situation that can be utilized to retain your existing numbers without much hassle.
Not that either of us can accurately predict what would happen to Black Mage dps in this hypothetical situation. I think it'd just make dps easier--and that seems to be what your post implies as well.
But either way, at the very least, were this to happen, you'd have a new cooldown to use that generates new gameplay options.
As for improving the base job's rotation, that's fair--but also outside of the scope of the thread.
