@Mojo Your The statement that "Slavery wouldn't still exist if was unsustainable and economically inefficient" is flawed. As an example, look at monopolies. Standard economic theory states that monopolies should be broken up in most situations because they, overall, produce less goods and services than a free market. Although it's quite thorny to explain here, you'll have to take my word that monopolies are much less efficient than an open market system. However, that doesn't mean monopolies aren't profitable. Quite the contrary, monopolies are hugely profitable...
but only for the individual firm. To the rest of society, it's extremely inefficient.
Again, what you're confounding is profit for an individual firm vs. most economically efficient for the entire market. Slaves are profitable. I agree. I'm not arguing with you on that. I'm saying that they are inefficient for the market as a whole.
I also want to address your point about China: the reason that it boosted with slave labor is because most of it's population was essentially unemployed. In other words, slave labor is still better than no labor. China was able to use its massive, and I mean MASSIVE unemployed farmer population EXTREMELY inefficiently (see slave labor) and still get amazing results because of the sheer number of people it has. Asian countries and developing countries can do this because in those countries, most of the population is previously jobless. However, in a more developed world, where most people already have a job, slave labor would be regression. Nor do developed countries have the population like China's to support slavery.
"BTW it occurred to me that "Slaves can't contribute to the market by buying or selling goods" well they ARE goods, so that's contributing to the market yes?"
Wrong, because that's part of the supply chain, not the demand chain. Essentially, slavery is a "cost" to the producer, since they have to invest (aka buy) a slave, therefore we consider it a cost. If you count that as part of economic activity, you're essentially double counting. Again, I won't get into the mechanics of this - you can read about this somewhere else - but that's why we don't consider, for instance, the cost of wheels in Tesla's car-making industry when accounting for GDP.
"Also free workers don't necessarily leave to "more efficiently be used elsewhere" For example, a person may be unemployed but not be willing to move across the country for an available job in their field. They rather work part time in an unrelated field to stay near friend and family. Free will is very inefficient."
Yes but most people, generally (of course), will tend to try and find the most high paying job they can. That naturally means that they gravitate towards the firm where their skillset is best used.
If you want to have an indepth conversation, I'm happy to explain to you any underlying mechanics or math to demonstrate the point. DM me and we can discord