" Slavery would have been a good thing in a society that allowed it so a person could work off a debt that they could otherwise not repay (meaning prison or perhaps forfeit their own life); or as a means to allow them to survive, if they were too poor to have food or shelter on their own, for whatever reason, including war."
That's not slavery. The idea you're trying to put forth is indentured servitude. Some thing a tad different from slavery.