The past decade was the worst for the U.S. economy in modern times, a sharp reversal from a long period of prosperity that is leading economists and policymakers to fundamentally rethink the underpinnings of the nation’s growth. It was, according to a wide range of data, a lost decade for American workers. The decade began in a moment of triumphalism — there was a current of thought among economists in 1999 that recessions were a thing of the past. By the end, there were two, bookends to a debt-driven expansion that was neither robust nor sustainable.