Labor Market Eventualities

"Now, I am leaning closer to my father's view. I think that a lot of structural changes are taking place that are driving the marginal product of many workers well below their opportunity cost. Their lost output is not missed terribly much, because productivity is doing so well elsewhere."
The Hobgoblin of Simple Minds, by Arnold Kling, Econlog

I have been thinking about this for a long time now. What do we do with so many people who cannot be efficient in the market place? Again, when manufacturing becomes as is predicted, cheap and incredible (here), how will most humans be useful?