But the reason they progressed out of slavery wasn't death but because free economies were far more productive and those which were more meritocratic than hereditary casted as well. Contrary to the popular "salty aristocrat origin" memes of new wealth only from exploitation and wealth only being ill-gotten they tried factories in the South Antebellum - they largely failed in comparison even with literal slave labor.
The point isn't how profitable slavery is or was. The point is what would happen if people who don't follow moral norms lived forever having accumulated a lot of wealth. If in this current system wealth is concentrated to a very small percentage of people, surely this would only increase given longer lifespans.
