It's simple. If one day we run out of people willing to work on goods for us for pittance, the price of goods we consume will rise and our quality of life will drop, because we won't be able to afford as much stuff. Right now, there are still billions in people living in poverty in countries that are friendly to capitalism (so, easy to set up a factory, a sewing sweatshop etc. there), so that risk is far from us.
I’m not convinced… there’s so much room for technology to fill the gap. Companies that fail to properly invest in tech to replace this labor will be beaten by those who do, and quality of life may actually improve as the marginal cost of production marches ever downward.
That's a vision of a potential future, not the world we live in right now. Right now, there are a couple hundred million people in Asia and Africa making things for the Western consumer, mostly for pittance wages.
It's mostly people in China, and as they've grown richer, their exports haven't gotten more expensive in the West. Which casts doubt on the whole hypothesis.
