I think we need to differentiate the possibility of slaver from the propriety of slavery. One is legally allowed to strike their son, its a Mishna, but very often it is an iffy proposition at best. Not everything that exists ought to be used.
Slavery exists. That is, the Torah recognizes one person owning another. Ought it be is another question entirely. The Torah recognizes a Jew being kidnapped and owned by non-Jews (v’yeshbt mimenu shevi, says the Torah). Does it recommend that or consider it particularly moral?
I think this distinction makes all the difference.