Credit downgrades can elicit fascinating reactions. Take a January move by Standard & Poor’s to cut Japan’s rating to the same level as China’s. I expected the backlash to come from Tokyo. Instead, it was the Chinese who were aghast. Every Chinese official I’ve met since is bewildered that 10 percent growth and $3 trillion of currency reserves don’t buy a better grade than the AA- China shares with an overly indebted, aging nation that names a new prime minister every year.