Well, there´s one more thing to consider: Is Nen really the only way to become (superhumanly) strong? After all, the Nen we know is more of a way for measy human beings trying to expand their might through unusual means. What if the Chimera-monsters possess some natural strength´s somewhat a counter power to Nen? Like, say, Rock Lee vs. Sasuke, Taijutsu vs. Ninjutsu. :> And then imagine the future King to combine both