They’re only useful if you leave the slave system.
They’re only useful if you leave the public school system.
If you never leave either, they’re only useful within those systems.
Leaving slavery is not a given. It was multigenerational. One of the things that made US slavery particularly evil was that you could be born into slavery and die in slavery and your whole families stayed in it.
Far distinct from individual trading which is a more standard form of slavery worldwide through history.
A slave class is a much different affair than most slavery systems