You neglected to include a good argument in favor of slavery.
Some people aren’t intelligent enough/don’t have high enough time preferences to function in modern society. Thus you either need to have them under the control of a master, or you wind up having to put them on the public dole and institutionalize the many of them anyway.
Well, in case you haven’t noticed aren’t in small forager tribes right now.
You’re right, I left out a few alternatives. We could also deport them to a haunter-gatherer society, let them go around engaging in tribal-style raids (although that tends to interfere with the functioning of modern society for those who can function in it), or let them starve to death.