Historically, some have speculated that with automation comes more and more leisure time, people not having to work because all of their needs have been fulfilled. What ends up happening in reality however (as we see now) is that productivity gains do end up with fewer people working but instead of more people working fewer hours, there are fewer people working more hours. What happens when there are not enough jobs to go around at all?
People won't have enough money to pay for goods. Will labor be parcelled out so more people work less? Will there be a perceived "non-need" for so many unemployed people? What happens then? I can't imagine it will be a pretty sight.