If we're using legal slavery as an example, countries that abolished slavery early ended up being more productive because employees actually want to work/innovate/create quality goods. By hiring slaves or treating your employees like crap, you're voluntarily specializing in the low-end of whatever market you're in, because your employees don't care about the product.

That's rarely the most profitable move unless you have a monopoly or a commodity good, like the examples you've provided.

We're doing this experiment right now in Canada by importing millions of temporary foreign workers/international students to work in slave-like conditions. If ruthless toxic slavery worked, we should be making more money. In reality, our GDP/capita has dropped continuously since 2019 indicating we're actually less profitable per person.