Krugman is a big fat idiot. If he wrote novels, they'd be dumber than Rand's. If he made movies, they'd be dumber than Michael Moore's.
Everybody talking offspring, percentages, and human ghettos can please get back to their real jobs. You are all seedling despots ready to sell out humanity, even if you don't know it yet.
The technology revolution we're up against is going to be so sweeping and the power and self-reliance of an individual will be so great that the question of dependency upon federal robo-credits or robo-biscuits will be moot. We will all be cyber-camping and living on impervious cyber-homesteads in fifty years. Energy, labor, and orgasms will be provided by your own fleet of robotic thingies. Technology beyond our wildest dreams will be as convenient as running water. We'll have figured out all kinds of awesome things about providing for our basic needs. In most of the world, the concept of inequality will be associated with totally bizarre things that our current value systems can't make sense of. (Kinda like how people in the year 1900 wouldn't understand why we fret over the right to internet access.) Everyone else will live in tacky McMansions and act like Real Housewives.