Ah, you ask, but what about the people? Very good question. 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.
And then eventually Skynet decides to kill us all, but that’s another story.
Interesting to see Krugman ruminate about these sorts of things.