> The part you’re leaving out is that, as a direct consequence of this practice, the South was a backwards and impoverished part of the country for that entire period.

The part that you're leaving out is that they were backwards and impoverished BY CHOICE. They could have automated just as much as the North did. They chose not to, because it was politically advantageous for land owners to keep their slave populations, which effectively also kept wages low for non-land owning white folk.