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.
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.