(Sorry about the title. The devil made me write it.)
What are we afraid of? Let's think about the worst-case, nightmare scenario for the distribution of income.
Assume that all capital is robots, and robots are perfect substitutes for human workers. One robot can produce everything and anything one human worker can produce. And that includes producing more robots.
And assume that every year the technology of robot production improves, so that it takes less and less time for one robot to produce another robot.
That sounds nightmarish, right? Because robots will get cheaper and cheaper, and drive down human wages?
Well, no. They won't. Or rather, it all depends. It depends on whether we add other forms of capital, or land, to the model.