This isn't true on a societal scale even though a few slaveholders built a bunch of grandiose mansions. Enslaved people were less economically productive than free people. It also locked economies into less productive, lower sectors like agriculture. The South resisted industrialization despite it being more profitable because it was incompatible with an economy built on minimally skilled slaves.

It was not profit that kept slavery alive well into the 19th century in the Americas and Muslim countries. It was something more sinister and evil.

The hard part is that I'm not sure any other system really fixes this flaw. Sure, you can be less democratic and give fewer people what they want, but for some reason few people want to live in autocracies of any stripe.

And it's not always clear that there is a solution when the things people want are too diametrically opposed, either. I'm not sure many people would be happy with any of the solutions from "Three Worlds Collide" for example (a short story you can go read online if you don't get the reference).

[EDIT: Added after that one person upvoted this comment.]

Slavery was big in the south because there wasn't enough low-skill labor to feel threatened. The north didn't care for it as much because a higher percentage of labor was what we would call semi-skilled or skilled today. That labor was FREAKED OUT about the idea of slaves taking over their jobs and they were able to organize before being eaten by the capitalist leviathan.

I'm pretty sure there were more abolitionists in the north than the south, I don't think your average northerner cared about the plight of southern slaves other than the institution being a threat to their livelihood if it moved north.

If you were making the assertion moral concerns or ethical behaviour eventually influenced american capitalism, I disagree. The capitalist monster acts "moral" or "ethical" because at the current time, to do otherwise is invite political dissolution. I fear the shadowy cabal of capitalist masters will move to reinstate chattel slavery. We did not respond with outrage when red-lining disenfranchised large portions of the populous or when usury was slipped back in with high credit card APRs and payday lending. We are asleep.

I’ve never heard of mainstream economics serving people. Maybe Keynesianism did?

it wasn't. Slave economies are exceptionally unprofitable and unproductive. To take the US as an example. Liquid wealth, i.e. capital, was vastly larger in the North than in the slave owning states, the industrial output of New York exceeded the entire Confederacy and it was that profitability, wealth and mechanized agricultural production that did them in.

Even Marx recognized this by the way, following feudalism capitalism was a progressive force, it was profit, productivity and surplus that enabled civil society, the north was more civil because it was rich and had unlocked modern forms of production. The problem of capitalism is not profit or lack of civil society.