You're still not listening. It's not bad for everyone individually, rather it's bad for society collectively. Slavery was obviously of benefit to the slavers.What you're doing is claiming slavery had nothing to do with US wealth and that it was bad for everyone.
Simple thought experiment: imagine one person working, they make one person worth of profit while the rest of the society benefits from one person worth of work. If you have 10 people instead, they each get their one person of profit while society benefits from 10 people worth of work. Now throw in slavery: you have one person who owns and directs the slaves and let's say they have 9 slaves doing all the work. If those slaves each do the same one person worth of work, but the slaver takes the profits, you have 9 people who have been robbed of their 1 person worth of profits, 1 person making 9 people worth of profit, and society only gets the benefit of 9 people working instead of 10. Even if the slaves accomplish half of what they would have as free people, the slaver still gets 4.5x what they would have otherwise while the rest of society gets half of the benefit.
Slavery existed (exists) to benefit specifically the slaveholder. It makes the slaveholder wealthy. But it does so at the full expense of the slaves and the partial expense of the rest of society. For the argument that slavery made the US as a whole wealthier to hold water, you have to believe the slaves accomplished more economically as slaves than they would have as free people, and not just more, but so much more as to overcome the sunk cost of the entire slaving industry. And there's no evidence of that.