Prison labor as a continuation of chattel slavery is seriously flawed. You may be arguing "prisons are worse than you think", but you're implying is that "slavery wasn't all that bad".
Prisons aren't profitable, they cost States and the Feds billions. It's true that private companies profit from prisons, but the vast majority of that profit comes from the prisoners and their families (often coercively) buying products/services.
White prisoners make up ~50% of the populations. Are you really going to argue that the Union's victory introduced white slavery to the US?
Prisoners have rights.
Incarceration isn't inherited.
The prison system is awful, racist, exploitive, senseless and unjust. But equating it with chattel slavery minimizes the abominable cruelty of that institution and serves white supremacy.