I don't know about greatest coach of all time, but I definitely agree he regrets leaving UK. I said in another thread that Pitino probably wins 3-5 championships if he stays at UK. But he wanted to try his hand at the NBA again. Failed miserably.
As for your second comment, fans vastly underrate Cal's coaching abilities, especially when he was at UMass and Memphis. He got each of those schools to FFs, and did not have the loaded teams he enjoyed at UK. Not even close.
For whatever reason, Cal got lazy in his last five years at UK. Whatever in game coaching ability he use to have was long gone. He was still getting loads of five star recruits, but when you are bringing your two lotto picks and best players off the bench, that tells you all you need to know about Cal.