To hear a number of prominent economists tell it, it doesn’t look good for the U.S. economy, not this year, not in 10 years. Leading thinkers in the dismal science speaking at an annual convention offered varying visions of U.S. economic decline, in the short, medium and long term. This year, the recovery may bog down as government stimulus measures dry up. In the long run, the United States must face up to inevitably being overtaken by China as the world’s largest economy. And it may have missed a chance to rein in its largest financial institutions, many of whom remain too big to fail and are getting bigger.