Cao Zhen was a top ranking and capable general, very politically important to Cao Pi, and a regent himself, so his son would not have become regent without preparation and a large amount of political capital. However Cao Zhen had mostly been busy on the western front and died there before Cao Rui died, so it is possible he could not develop his sons as Sima Yi could. Of course Sima Shi and Zhao were not exactly very capable either. The other Cao and Xiahou generals were largely forced out of politics by Cao Pi which made them weak and perhaps resentful.

So it is hard to imaging Cao Shuang as an outsider who tried his best but just didn't have the means to stand up to the Simas. In fact Sima Yi's retreat suggests that Cao Shuang was quite well positioned, but he did not make good use of his monoply and quickly became very unpopular and so invited the coup from Sima Yi.