Henry Clay’s nephew, an abolitionist, criticized slavery on purely economic terms. He claimed that slavery prevented the economic development of the South. The Civil War certainly did not help.
It was cheaper for a capitalist in the South to invest in slaves than machinery and the availibility of slaves (or illegal immigrants today) depressed wages at the low end of the pay scale.
Cotton, of course, became the king of the crops that underpinned the South’s economy. Cotton wasn’t the cause of slavery, slavery came first, but with the development of the cotton gin it made slavery favorable as a low-cost method of harvest. This then was a major part of the South remaining a relatively backward, agrarian region. While several of the Southern states were among the most wealthy, that wealth remained with the slave owners and was used by them to expand their operations instead of finding its way into social growth and development.
Cotton also meant faster soil erosion, so the South’s insistence on westward expansion can also be seen in that light. There is far more to this than can be added here in a few words. But I wonder what Henry Clay, a slave owner, thought of his nephew’s position. I’ve not heard.