I tend to disagree with that somewhat. Slave labor is not enough, since if it were, we would still be living in slavery. It is evident that you refer to slavery metaphorically, but real slavery ended not because there were no more slaves left, but because their labor was ineffective compared with hired workers and free farmers. China just happens to have huge population and therefore high competition on the labor market.
It's a race to the bottom, that's for sure. I'm all for free market capitalism and free enterprise, but what you have in China is a bizarre mix of Communism and Social Darwinism: you have the elites at the apex of the pyramid with all the slaves at the bottom. It's modern day slavery, in the sense that you work 10/12 hours a day chained to a work desk for a pittance of a salary. If that isn't slavery, I don't know what is.
But you're right, multinationals are slowly moving to other countries with better access to slave labor. Will the same thing happen with Bitcoin? If miners start exploring other options and diversifying to the Philippines or Bangladesh, this might actually be a good thing