Whenever a person proposes that technological advances might reduce human job opportunities in the long term, someone responds with the Luddite Argument: "the Luddites thought the Industrial Revolution would destroy their jobs, and they were wrong, so you're wrong too" . Some go further and explain that the Luddites were wrong because technological productivity improvements are balanced by finding new uses for human labour. Wikipedia has a good summary. However, it seems obvious to me that at some point technological advance will --- or at least could --- be a net destroyer of jobs. All you have to do is imagine a world where robots can do everything a human can do, at lower cost than maintaining a human life. Clearly, there are no economically rational job opportunities for humans in that state , so at some point of technological advance short of that state, there's net job destruction.
The only question is whether and when we will reach that point. It seems inevitable we'll reach it unless something halts technological progress or some very strong flavor of Cartesian dualism holds. Economic arguments that human labour will still be worth something at that point are just wrong.
 Actually the Luddites were right; the Industrial Revolution did destroy their jobs, and drove them into misery. But they were wrong in that they did not forsee the net benefits to future generations.
 There could be sinecures to keep humans occupied, but they would not be economically motivated.