Why not try to prove that he can be top-notch at the NBA level too? He's still fairly young. If he fails in the NBA (again), I can assure you, any team in the NCAA, except the Ivy League schools and Vandy, will take him.
I can see that to a degree but still have a hard time seeing him leave from where he is at right now. If Calipari heads to the NBA again it should be anyone BUT the Knicks. The entire Knicks organization is a mess from top to bottom from the administration to the players. Could Calipari handle it? Most likely. But it would be a couple years before they could ever have some success. The contracts with Carmelo and Amare really leave them in a bind with signing other players so I just have a hard time seeing him being successful there.