Just consider this: in today's society a significant proportion of people (US citizens) are out of work.
It's not that they are useless trash
The reasons they are discarded vary.
Mostly it's competition from within. Companies always shop for the best price performance ratio. In production machinery, printers, staples, and employees. So they sort applicants and current workers by price-performance ratio, and try to make their workforce structure resemble as much as possible the optimum available in the job market. Through hire and fire policy. Maintaining that "best match" with the labour market is the main reason companies have an HR department. No hard feelings, just business.
Competition can also come from outside though. Examples are H1B visa and illegal immigrants from Mexico. Please note that there could never have been any issue whatsoever with illegal immigrants if employers weren't prepared to employ undocumented applicants. But they are
Approximately the same holds for automation. Throughout the ages, as technology advanced people were expelled from one type of function (e.g. agriculture, manufacturing, mining) and had to seek employ in another function (farmers becoming labourers, labourers going to work in the service industries, etc.). An example is the industrial revolution. Historically that has led to a massive shift in the job market (farming to industry), unemployment, a large drop in wages, terrible working conditions, misery, and widespread exploitation of people by employers. Society finally regained its equilibrium after a century or so, in part due to the threat of revolution.
The only difference is that the current technology is poised to make certain groups of people uneconomical to employ. It's not just that their jobs disappear, it's jobs of the kind they are capable of doing become prone to being automated.
Take the 6 mln. or so truckers.we have now. We can replace one third of them with self-driving trucks, at huge benefits. Now what other work would somebody who likes being a trucker be good at? Not sitting indoors and shuffling paper I suppose.
Take the car industry. Plants today are highly robotised. Cheaper, better, more flexible. More automobile workers surplus to requirements. What type of work would they be good at? What kind of work are they trained for?
Take scores of people in administrative functions like the insurance industry. Doing administration and processing claims can increasingly be done by software. AI or not. Lets replace them. Miners (remember those hopeful Trump voters in mining villages) are on the way out because coal is being pushed out of the market and not coming back.
Take ready made products. Those can be made far cheaper abroad and then shipped to the US. Despite the little temper tantrums by Pres. Trump and his supporters it's not economically feasible for the US to stop that. Other economies would overtake the US and start dominating it. So it's probably not going to happen to any meaningful degree for any meaningful length of time.
The list of labour displacing developments goes on. And on.
All this wouldn't be a problem if we could readily think of other (paid !) work we could let the freshly turned-surplus-to-requirements workers do. But can we? Really?
I don't see it and I'm no longer optimistic we will think of something genuinely new.
In any event, we have limited options to respond.
We could delay or even *temporarily) halt the economic mechanisms that push workers into the surplus bin. And cut our own throat, economically speaking.
We could simply tell the unemployed to drop dead. Dangerous. Gives civil unrest, increased crime rates, and allows even more people like Trump and Bannon to float to the top.
We could provide unemployment benefits for those discarded by the labour market and put them into storage until they're needed.
By and large I think it will be option 3.
If just about anybody but Bill Gates had put this idea forward, they would have been trounced as socialists, pinko's, bleeding heart liberals, etc. When Mr. Gates says it
I'm sure that a tax on robots is something that can easily be done in ways that cause more harm than good
It wouldn't be the first time he saw a trend that everybody else didn't (until it was upon them).