"Oxen? Paid laborers? "
In other words, animal and human muscle, we agree on that.
I didn't claim that all human labour was non-free, far from that. Every classical civilization had paid artisans and employees as well.
But the paid professions tended to be the skilled ones, and the non-free ones tended to be the arduous, backbreaking ones.
"The South was notable in clinging to slavery long after it had been abolished elsewhere."
Elsewhere where? If I look at the timeline of slavery abolition on Wikipedia, it seems that the South was not even the last holdout in the Americas, much less worldwide.
https://en.wikipedia.org/wiki/Timeline_of_abolition_of_slave...
They were about as delayed as Russia. (Serfdom in Russia was not quite slavery, but brutal and backward nonetheless.)
And the timeline of slavery abolition seems to dovetail with the expansion of the Industrial Revolution across the globe quite tightly, or not?
"it was at least in part cultural."
Chicken, egg. This is a system stretching over millennia with endless feedback loops. Runaway slaves may become the masters (such as the Aztecs) and vice versa, developing their own justifications why it happened.