Smart machines may make higher GDP possible, but also reduce the demand for people — including smart people. So we could be looking at a society that grows ever richer, but in which all the gains in wealth accrue to whoever owns the robots.
Now I know Paul Krugman is a science fiction fan, so in the interests of exploring his speculation maybe he should read the Culture series from Iain M Banks. In the meantime:
A study commissioned by the tech advocacy group TechNet found that the “app economy” — including Apple, Facebook, Google’s Android and other app platforms — was responsible, directly and indirectly, for 466,000 jobs.
That's half a million jobs we'd not even thought about five years ago. A real value-adding jobs too - jobs created entirely by new technology.