Re: "Somehow, the work has always expanded to use most of the population of working age."

Machines are still very stupid in many work-related domains - relative to humans. The issue of what on earth the unemployed will do is likely to arise with greater acuity once machine capabilities shoot past our own in most industry-related domains - retail, farming, distribution, mining, etc.

I go into these issues on: http://alife.co.uk/essays/will_machines_take_our_jobs/