You kinda missed the whole point of the OP, which was, if people don't add value to a business, how will said valueless people put food on the table and participate in society?
Is it rough when (for example) shipping crates put a bunch of longshoremen out of a job, when they are in their 50s and too old to easily retrain? Sure. But how many people now not only enjoy (as customers) the benefits that Amazon provides, but also enjoy the employment benefits of a massively expanded global market?
On the other hand, the alternatives are harsher: as the population grows and people resist the very innovation that could create new markets, unemployment will grow. For all the problems we have in the US, providing low unemployment is not one.
Those newly opened industries have less people in them than the industries they replace. Mostly because our consumption doesn't increase as much as our efficiency does.
Funny you mention Amazon. We enjoy their services, and they enjoy that expanded global market. And they do this by killing local businesses with their ruthless efficiency. (Remember we're supposed to be competitive. There will be winners and there will be losers.) They don't create nearly enough jobs to compensate for the unemployment they cause.
There's also the problem of skill. For instance, in 20 years, truck drivers will be gone. Instead, we'll have an operator managing a fleet of 50 trucks from some control centre. The rest will have to find something else. But what exactly? The low skill jobs will be even scarcer than they are now, and the high skill jobs will be taken up by people who enjoyed a higher education in the first place, as they are now. You could still raise the level of education, but that will be too late for those drivers.
Keep this up to its logical (and absurd) conclusion, and half the population will starve to death in the streets. They're useless, why the market would support them? Of course, we'll have cleaner robots wipe it all away, cleansing the stench of death —those who still have value will be able to enjoy their walk in the park undisturbed.
More realistically though, we'll have a revolution before we get to this point. Or, the rich and powerful will see it coming, and start giving bread and games to the people. Come to think of it, they already do it to some extent.
I'm not saying that people who are displaced will easily be able to retrain to a new job. They will have a rough time, and that's something we need to address as a society.
But assuming that number of "jobs" for humans will decrease to zero is a big stretch. I think that even if we had robots that could take care of all our needs for us, humans would come up with jobs to occupy their time.
It seems to me the amount of available work is steadily decreasing right now. Perhaps not towards zero, but we don't have to lose that much to need substantial reform. Once you reach over 30% unemployment, you have a big problem already.
There's an easy way to increase the number of jobs though: share the existing ones by reducing the work-week. We could start by shaving off a few hours, perhaps as much as a full day (for a 4 day work-week). Then decrease it further to match progress in automation. Of course, we should keep the salaries at the same levels, lest machine owners will just get a bigger and bigger share of the pie, just because they had starting capital the other workers didn't.
We've already essentially phased out the useful work that people under IQ 70 can do. It's likely that it's creeped up to 75 or 80, even. As these are automated, let's creep it up to 90.
It also takes out those who are poor and have no branch on which to gain experience for their first job. If we have eliminated the first jobs for all but the most talented and those with connections, what does a somewhat bright poor student do?
Well, it used to be they got a job, proved themselves, and worked their way up. Now, what do they do when they don't have the social capital to get to the point where they can work up?
What are these newly opened industries that will work with these people?
As far as I can tell, it's "magic future ones" and the evidence for these statements is "because I hope that's what happens".
If that really is the best answer then it's time to invest in guillotine manufacturers.
The scary part is it looks like wealth will be increasingly concentrated, and more and more people will fall to a level where what we consider basic needs in modern society (food, shelter, education and health care) will no longer be attainable. As the author points out, "it will be messy": Those societies usually don't end well.
A diminishing pool of people to pay for stuff, not only hurts businesses profits, but also diminishes the pool of rich people (who make their money off of selling products, houses, services, etc to the masses).