There is absolutely no law of economics that says that wages can't decrease. Wages are based on the supply and demand of labor, and there is no question that robots will affect that. If the wages fall below minimum wage, or even what it is feasible to survive on, then you get unemployment.
Most human jobs for most of history have been unskilled work. Things anyone could be trained to do, not requiring high intelligence, and involving physical or repetitive work. The industrial revolution moved unskilled laborers from farms to factories, but there were still jobs.
But with near future tech, there will be very few things, if anything, unskilled humans can do that robots can't. We are just starting to see AI good enough that it can match humans in visual recognition tasks - which 10 years ago was unthinkable, and progress incredibly slow. This level of AI has been the main limitation to robots. We've had decent robots, but AI wasn't good enough to use them for anything interesting. Now it is.
Soon robots will take over almost all existing jobs. From truck drivers to shelf stockers to the mail man.
Of course there will be jobs that AI can't do (for now.) E.g. computer programmers or doctors. But realistically, what percent of the population can actually do those jobs? What percent of unemployed truck drivers are actually going to be able to retrain as computer programmers?