I have thought of how interesting it would be for a FF game where you play as the Warriors of Darkness. As you say, there needs to be balance. What would a world overcome by light be? Would it persecute anyone who made a single sin? Would the world become grey, and dull?

But currently, Eorzea is not such a place. Darkness is running pretty freakin' rampant. We kind of need every warrior of light we can get.

Eventually, there might be a FF game where darkness needs to right the balance. But it is not this game.