This, oddly, is the answer to your question. If the heroes in a series always win, it's not a story, it's on-going episodic. Dark must have an obvious advantage for the sheer purpose of making one wonder who on the light side will survive. This is why, typically, in stories where "the devil" is present, "god" is not (or the two turn out to be twin brothers). It is because "good wins from the start" does not make a good story. "Does good win? Let's find out." does.