Oh no. Human rationalizations for earthly power are just FINE!
Give me a break. Anybody who thinks John the XXIII through BenXVI are interested in earthly power is delusional.
So is Jesus really a cornerstone? Is He granite or gneiss or what?
How can we say it more explicitly? It's a metaphor, a figure of speech. When Peter is called the Rock on which the Church is built, Jesus presents himself as the builder. Who is the builder in the Ephesians metaphor? -- it's a different analogy.
There is no intellectual problem in saying "In THIS way, Peter is a rock; in THAT he isn't, but his faith is, or Jesus is." And that's essentially what the CCC does. The Ephesians text doesn't vitiate the Matthew text (or require a strained parsing thereof), it just shows that both are analogies.
So Peter is not the rock?