I'd say it's at least clear that so far automation has caused little to no long-term unemployment. (Again, the industrial revolution started a couple centuries ago and yet there are still jobs.) Generally what happens is:
Edit: Take agriculture as an example. Wikipedia says that about a billion people (1/7 of the world's population and "over 1/3 of the available work force") currently work in agriculture. That article doesn't give versions of this worldwide number for previous points in history, but it does say, "During the 16th century in Europe, for example, between 55 and 75 percent of the population was engaged in agriculture, depending on the country. By the 19th century in Europe, this had dropped to between 35 and 65 percent. In the same countries today, the figure is less than 10%."
spend their extra money on other things (meaning more employment is available in other industries for the former widget makers)
That doesn't follow. It may mean that more jobs are available in other industries but these jobs require skills that the former widget makers don't have, thus creating an imbalance where one area of the economy has reduced unemployment and one has increased unemployment.
Furthermore, it doesn't mean that the number of jobs is balanced out, but the amount of money. It is entirely possible that three jobs worth $X are lost among widget makers and one job worth $3X is created in the other area of the economy.
I haven't given much thought to the concept of automation and computer induced unemployment. Others at the FHI have been looking into it in more details - see Carl Frey's "The Future of Employment", which did estimates for 70 chosen professions as to their degree of automatability, and extended the results of this using O∗NET, an online service developed for the US Department of Labor, which gave the key features of an occupation as a standardised and measurable set of variables.
The reasons that I haven't been looking at it too much is that AI-unemployment has considerably less impact that AI-superintelligence, and thus is a less important use of time. However, if automation does cause mass unemployment, then advocating for AI safety will happen in a very different context to currently. Much will depend on how that mass unemployment problem is dealt with, what lessons are learnt, and the views of whoever is the most powerful in society. Just off the top of my head, I could think of four scenarios on whether risk goes up or down, depending on whether the unemployment problem was satisfactorily "solved" or not:
with AI problems, people and
organisations are willing and
able to address the big issues.
misery that unrestricted AI
research can cause, and very
wary of future disruptions. Those
at the top want to hang on to
their gains, and they are the one
with the most control over AIs
and automation research.
automation problems in a
particular way (eg taxation),
people underestimate the risk
and expect the same
solutions to work.
conflict between those benefiting
from automation and those
losing out, and superintelligence
is seen through the same prism.
Those who profited from
automation are the most
powerful, and decide to push
ahead.
But of course the situation is far more complicated, with many different possible permutations, and no guarantee that the same approach will be used across the planet. And let the division into four boxes not fool us into thinking that any is of comparable probability to the others - more research is (really) needed.