If the human labor force is wiped out & replaced by robots, who's going to be able to buy anything? Industry is what supports the middle class. Nobody seems to be talking about how the economy is supposed to work when there is very minimal need for human labor.
There is already global competition for jobs which is why Trump bringing industry back to the USA is hailed as a good thing. Though that also means fewer jobs in countries we're insisting on the right to export to. Canada for example is doing terribly with full time jobs not paying enough to let workers afford even an apartment on top of food, and Trump is cheering about industries there relocating to the states.
So if there are not enough human jobs globally right now. the intro of mass robot labor will make it much, much worse.
This is what the UBI is theorized to address. In a futuristic George Jetson world where everything can be provided at no cost, why should anyone have to pay for at least basic needs? What's the answer to that?