If slavery was the basis of successful capitalism; why did the South lose the war? It was the industrial power of the North. Capitalism is based on Industry not agriculture; ask Karl Marx
The Industrial revolution was not based on slavery; therefore Capitalism was not based on slavery.
The claim is not that slavery itself was profitable or workable over the long haul (I think industrialization and mechanization would have killed slavery in the US in a few more decades without a civil war), but that fortunes made in the slave trade/triangular trade itself, from slave-produced sugar, then from slave-produced cotton during the ante-Bellum period, were critical to the capital accumulation necessary for the Industrial Revolution, in which cotton textiles were a key element. Britain’s global empire and economic domination arose from that base.
Much statistical and historical evidence for this can be brought to bear. My problem with it is that obsession with one factor crowds out proper consideration of others. There is also the problem of why slavery in other contexts produced other results (it stifled technology in Rome; it produced massive amounts of precious metals in the Spanish Empire which became moribund).