As for the claim that the US economy grew after slaves were freed, that's probably true but a little something called the Industrial Revolution also came at that time.
The Industrial Revolution happened in other countries too, that did not have slavery to start with, like Sweden or Switzerland. And those countries were richer than the Southern US states back then, and even are richer than them today - both the poorest are richer AND the richest are richer. The places in the US that benefited from the Industrial Revolution most were industrializing even before the Civil War.
Slavery is an expensive thing to operate. You need infrastructure to buy and sell slaves, guards to make them do absolutely anything and ensure they aren't running away (and even in HH, they seem to be quite willing to run away, Nia appears to be quite prolific in her slave-breakout operations), and you can only get them to do so much. Also, a lot of what slaves do can be automated - that, too, is also happening even in HH, where slaves seem relegated to being servants and sex toys, and even there Kali's gonna automate them out of even that job.
modern slavery consists of chinese wage slaves working for scraps to make things cheap for the US.
Terminology of "wage slavery" and mechanics of how it arises aside, that doesn't make a powerful economy. China is actually developing out of that stage, picking up more advanced manufacturing, and their workers are actually starting to run out and be paid decently well. The image of "chinese wage slave working for peanuts" is from the 1980s, not 2020. That stage in economic development is now relegated to places like Bangladesh, India, Indonesia, or Vietnam - hardly the most economically powerful nations. Because any form of slavery just doesn't make for a powerful economy.
An army of warrior slaves makes you powerful
Heyo, Spartacus called, asked me to tell you that an army of warrior slaves makes a massive rebellion. You never ever ever arm your slaves. Most you ever do is use them as barely armed cannon fodder. Unwilling soldiers are soldiers that are likely to defect to the enemy just to spite you. In fact, oppressed population are extremely willing to take up arms and fight their oppressors just out of spite, even if not given weapons by their oppressors (which like, happened so rarely if ever that I literally can't think of a single moment in history where someone took unwilling slaves, gave them weapons, and forced them to fight) - also happened in the US civil war.
Whenever slave armies happened, or even such things as the draft happened, there was some actual incentive to fight. They don't fight under threat, they fight to become free (Mamluks) or to get access to important government offices (Janissaries) or similar. If you threaten your soldier they're likely to just turn their weapons on you
If you have 1,000 people mining gold for you as you only pay them in bread, do you really think you're not going to become wealthy?
Considering that you need a guard for about 7 slaves, you wouldn't become as wealthy as you'd become having 10 miners with dynamite, drills, and excavation machinery. Not even as wealthy as you'd be if you took that money that you give to the (quite expensive) guards, and the money you spend on slaves, and just gave that to 1000 free workers.
In general, while I take a lot of issue (and I mean a LOT of issue) with the politics of the game, I don't wanna be too nitpicky, and I am letting most political messages slide under the implicit "it's fiction." response. But economics of slavery is one thing I just can't allow to slide. It is important to me that people understand that slavery just doesn't work, that people view slavery in the same way as they view bloodletting - a horrifying practice that doesn't even work. That they not only oppose it on the basis that it's horrible (since some sociopaths might not care about that), but that it's also stupid and just doesn't work.