I find it tough to accept modern views that truly severe crimes can potentially be punishable by death, but certainly not by slavery. In all fairness the death penalty is primarily an American institution at this point (other countries have moved on), but I’m willing to go as far as to say severe incarceration (like solitary confinement) is still morally worse than enslaving someone — forcing them to labor for the public good under reasonable living conditions. It seems like the idea of slavery, at least in comparison to an out-of-sight imposition of mentally ravaging solitary confinement, would be far more publicly noticeable and likely trigger some form of repugnance. Thomas More appears to agree (from Utopia), although back then the repugnance was sought out:
For the most part, slavery is the punishment even of the greatest crimes; for as that is no less terrible to the criminals themselves than death, so they think the preserving them in a state of servitude is more for the interest of the commonwealth than killing them; since as their labor is a greater benefit to the public than their death could be, so the sight of their misery is a more lasting terror to other men that that which would be given by their death.