I don't think it's quite true, as other people have stated, industrialization (for example in China) indeed helped almost everybody, not just the subset of the

I don't think it's quite true, as other people have stated, industrialization (for example in China) indeed helped almost everybody, not just the subset of the wealthy or middle class people. I think it comes down to this: Upward mobility is a little misleading term, because you can increase it not only by exploiting more slaves (as you seem to think), but also by reducing class inequality itself. As inequality gets smaller, the distance to travel (in terms of social mobility) decreases, and more people can cross classes. And I would argue, what the West managed to do in the period of circa 1930-1970, was indeed a massive reduction in social inequality. This happened not by chance, but was a result of many years of struggle for social democracy, starting from the French revolution (rejection of nobility as a concept), and continuing in pursuing socialist ideas in the late 19th century, and rise of the unions in the workplace.