Why didn't Cao Shi kill Sima Yi and let him take power instead?

Rights are acquired bit by bit. Cao Cao doesn't need Sima Yi just to guard against him. Before he died, he advised Cao Pi not to use him. Cao Pi's talent was far less than his father's. Although it was not reused, it did give Sima Yi a certain space to display his talents. It was Cao Rui and Zhuge Liang who directly put Sima Yi on the historical stage. Zhuge Liang made many northern expeditions, and Wei was forced to use Sima Yi, which made him enjoy prestige in the army. After Zhuge Liang's death, Cao Rui was relieved, ate, drank, and gave all power to Sima Yi and Cao Shuang. And Cao Shuang is a waste, which enemy is stronger than Sima Yi?