No one ever said that. The prophecy isn't "There are too many good guys. There should be an equal number of good guys and bad guys." It isn't about what should happen. The prophecy isn't recommending a course of action, merely an event that will transpire. A prophecy doesn't have to be a positive thing that people should strive for. It might be something people don't want to have happen at all. Oedipus fulfilled a prophecy that said he would kill his father and marry his mother, and thus brought disaster on his city and family.