We all seem to be worried about the robots taking over these days - and they don't need to take all the jobs to be horrendously disruptive.
A situation where 30 to 40 per cent of the working-age population were economically useless would be tough enough. They might be taxi drivers replaced by a self-driving car or hedge fund managers replaced by an algorithm. By "economically useless", I mean people unable to secure work at anything approaching a living wage. For all their value as citizens, friends, parents, and their intrinsic worth as human beings, they would simply have no role in the economic system.