Cao Cao advocated "talent-only action". Why did Cao Pi implement the "nine-grade system" after he took office?

many history enthusiasts believe that it was the "nine-grade system" implemented by Cao Pi that gave Sima a chance to usurp the throne. And Cao Pi father Cao Cao, in the selection of talent standards, "wayward" breakthrough in both political integrity and talent and pay attention to talent-only. From this, it can be inferred that Cao Pibi's father was both stupid and lacking courage. Such comments as

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