The thing that is interesting about this argument is that it is the same argument that people have had since the rise of the Industrial Revolution 400 years ago. It crops up at least once a generation. Malthus thought that we were heading to a population crisis and advocated for what we now call eugenics to solve the "poor problem" of his time. At the time we was alive, there were fewer than a billion humans alive.

Let's say everything is right, and machines can automate 60-70% of the workforce.

As anyone who has worked on a big project can attest to, the first 90% of the work is easy and goes quickly. That last 10% to complete the work is full of unforeseen issues, problems, disasters and other bits of fun and excitement. I was told that 90% of your budget is spent in the last 10% of the job, and I guess this is a good rule of thumb to go by. As stated below, computers and machinery do some things much better than humans will ever be able to do. And there are some things that humans will always do better (language and interpreters for example are based as much on facial and body language as much as the words themselves).

Computers are creating whole new industries that we are not able to see right now. 3D printing is going to destabilize whole industries and economies while creating millionaires who are able to look at this and say 'this solves a need that is not being met." Cars created a need for malls and big box stores, diesel engines made transporting cheaper and more reliable, computers have made just about everything cheaper, better, stronger.

The worst case scenario that I see is that there will be a further decoupling (love that word!) of productivity and salary causing a drop in consumer demand due to less cash to spend on stuff. Land and real estate will continue to soar in cities as real physical goods are something that everyone understands and cannot really be created. But people can only pay $4000 a month for an apartment in San Francisco for so long before they go "Fuck everything about this, I'll go make 100K a year less and live in a house on 3 acres in Nashville." It is going to take time, but the adjustment is coming.