It's a common trope. The logic here is to keep identifying and eliminating the demon king candidate before it fully develop as the demon king so it won't be a threat to human race. What we know is they identified the hero early enough, and we don't know the ones who are issuing the orders and what their end goal is.I think the author's reasoning has a fatal flaw. If the successor to Demon King is born every time the previous Demon King succumbs, I don't think eliminating that successor is going to do very much to alleviate the situation...