Because if productivity can be achieved with less people, and even intellectual work is not protected from machines, people will become redundant for the creation of economic value.
Productivity is achieving more with the same number of people. The end result is more stuff (goods/services) at a lower price. It seems unlikely that this will be bad: wealth = f(technology)
Optimists sound dumb in the present but are usually right in the long run. Pessimists sounds smart in the present but are usually wrong and miss out on opportunities.
If not, which year in the past was significantly better than today? What was humanity's high water mark? I suspect we are nowhere near it.
