I hear this point of view, that mass incarceration is slavery with extra steps. But a (very quick and not in depth) google search shows that the cost to keep a person in prison for a year is $60-100k. And as far as I can tell in the cases where prisoners are laboring, while for admittedly shit pay, its not in $50k/year let alone one where where a profit would be returned for the "owners/jailers"
Now, if you're saying that the slavery comparison is more in that prisoners are legit balance sheet items for private prisoners to collect tax money? Well there is an argument there to be sure. But this seems like a structural problem. The existence of private prisons at all.
None of that is arguing in favor of crazy sentences for non-violent crime, however directly comparing it to chattel slavery confuses the argument against mass incarceration.
Prisons and more specifically prisoners are massively profitable to their jailers/owners no matter if they're private, state, or federal. Even private corporations are profiting from the US prison system. The federal government runs a corporation called FPI/UNICOR which sells out prisoners for profit. States also have programs or companies that lease their prisoners out to private companies who pay those prisons (or the government) a lot of money to get an endless supply of workers who can't complain about working conditions or being paid pennies a day (almost all of which goes back to the prison anyway). Some prisoners are forced to work for no pay at all.
Prisons will let people who are supposedly so dangerous that they need to be locked up and denied parole out of jail long enough to work their shifts at fast food restaurants, retail stores, plants (like Tyson Foods) and warehouses and they'll run their own companies inside the prisons themselves like call centers which get contracted out to private corporations or government agencies for a profit.
Some prisons also run farms, modern day plantations, that use slave labor to sell millions of dollars worth of food and crops to corporations whose products show up in our stores. Whole foods, kroger, walmart, and target all sell products made by prison labor. Food and crops raised on prison farms even get exported to other countries.
Inside the prisons everything prisoners get they have to buy from the prison which overcharges them for everything. The workers make pennies but the markups on what they need can be 600%. The commissary vendors are for-profit and the prisons get kickbacks from them to get exclusive access to the captive customer base. Phone calls from prisons can cost over $20 for just 15 minutes (https://brilliantmaps.com/jail-call-cost-usa/). The US prison system is designed to make money. Private prisons tend to make more profit on their prisoners, but so do state and federal prisons, the vendors they employ, and private corporations.