I think more recent scholarship throws such assumptions into question, and paints a picture of the South as highly energetic, ambitious, risk-taking and most strikingly, prosperous.
Yes, Southerners had fewer miles of railroad and fewer factories than the North, but only the North and parts of England exceeded them in that.
And no nation, including the North exceeded the Deep Cotton South in (white) wealth per-capita, when you consider the accumulated value of slaves.
One thing antebellum Southerners did freely admit was their own unsuitability for field work in their hot summer climate.
That's why, they said, they needed & could not do without African slaves.
Bottom line: from, say 1830 to 1860 Southern growth in population and economics compared to that of the North, when you equate Southern plantations with Northern factories.
And, as many FR posters have noted, by 1860 at least half and maybe 3/4, of US exports (depending on how, exactly, you count it) were products of the South.
So the South was more dynamic that perhaps people like your Thomas Sowell give them credit for.
I think equating factories with plantations and industrial capital with slaves is what we call a “category error.” They are different economic units. The more useful comparisons would be of Northern vs. Southern farms and Northern vs. Southern factories. Compare output, productivity of capital and labor inputs, production diversity, etc.