Someone else phrased it best, I read it from an HN user but they may have been using someone else's argument. Paraphrasing here:
"Machines made physical labor easier, requiring fewer people to do physical labor. Those people moved to doing mental labor. Now machines are replacing mental tasks too. Where can people go when fewer of them are needed for mental tasks?"
The last remaining type of labor would be creative labor. Writing, music, performance. Things machines either still struggle at or won't be able to perform until we have humanoids that move fluidly. Even then, humans seem to prefer humans performing "artsy" things - based on criticisms I've seen of robots who create art (mostly music and drawings).
Art also, notoriously, doesn't pay very well for all but a small fraction of artists. What happens when this market is over-saturated because it is the only job left for large parts of the population?