Capitalists are selfish, and have short planning horizons. For a capitalist, slavery is absolutely rational. The only reason why they don't do it today is because it's illegal.
Which is why it was the pseudoaristocraric south which provided the main source of abolitionists and the first to abolish it. Is what I would say in opposite world.
Selfish capitalists want to accumulate capital and grow the economy. Slavery is a bad strategy for achieving these ends. There is ample historical evidence of this, and it also lines up with mainstream economic theory.
People who argue otherwise simply cannot escape their zero-sum worldview.