There is a famous, but no doubt apochryphal tale, about Henry Ford and an assembly line worker. Ford tells the worker that he will soon be redundant. Henry has bought some new super robots. They work faster than the worker, 24 hours a day, 365 days a year, and never go on strike. The worker says to Henry "Very good Mr Ford. And will they buy your motor cars?". What the fictional worker understood, and the fictional Henry Ford had forgotten, is that economics about supply and demand; about production and consumption. [This is always a good story t0 tell engineers - they tend to be obsessed about production and not to worried about consumption, marketing, sales, advertizing and all that "softy" stuff.]
The theoretical basis of the above story is, of course, best understood by using the analysis of Karl Marx. In brief, the circuit of capital in capitalist society fundamentally relies upon the production of value (e.g. in manufacturing). BUT that is only one moment in the circuit. The second moment is the realization of value (e.g. selling the products made in phase one) and this is followed by the distribution of value (e.g. dividing this up in dividends and increased share prices). When this distribution is then re-invested the next circuit of capital begins and capital accumulation continues. All three moments are required for capital accumulation. In other words, capital is inescapably tied to labour; the former cannot grow without the latter.
The bad news is that, in a globalized world, there is no guarantee that US/EU capital is reliant on US/EU labour. Workers in the UK might be impoverished and workers elsewhere become the new important consumers. Nevertheless, capital has to find markets and that has to mean consumers who are able to buy the goods and services produced by corporations.
Capitalism is ever changing, so the delicate balances in the circuit of capital, and the relations between different elements of the circuit - production, retail and finance branches of capital - are repeatedly throwing advanced capitalism into crisis - e.g. 2007/08.
Edit: I realize that I have left out a crucial element in the analysis. Where does the value come from? For Marx, it is the result of human effort - labour - applied to natural or cultural materials. When the amount of value that is returned to labour is less than the value that is created we have surplus value which, under capitalism, is appropriated by capital in a process of exploitation of value. This is why capital relies upon labour. [NOTE: many conventional, neo-classical economists reject the labour theory of value].
