I posted this in Pets/ Domestication , and am moving it here:
A slave-dependent society only really works when there is a regular influx of new slaves.
The purpose of the slaves is to perform labour at a lower economic cost than regular workers (considering requirements for food, clothing, shelter, etc.) The slaves are provided with less than they require to sustain themselves long-term, thus working themselves to death, and requiring that they be replaced.
Once the availability of new slaves decreases, continuing the use of slaves means giving them enough to survive for longer and reproduce effectively. This means raising their living standard, and using more resources on them. At that point, they become indentured servants, rather than full-on slaves. And if the system relies on heavy use of slavery, it can no longer function in the same way.
It’s still a form of slavery, either way. And there is no reason why a servant necessarily couldn’t be owned by their master. Until the Nineteenth Century, married women in Britain were officially owned by their husbands (except for royals, and such). There are many forms of slavery, though some are less extreme. My point was that when the conditions which form the basis of a society change, that society cannot continue to exist in the same way.