Does the positive side of tribalism/racism outshine the negative side?

[Greatest I am]

Descriptive moral statements refer to how things are or how things have been, not how they should be or whether they should be.  Your disagreement is with the latter, not the former.  

Your question assumes that slavery is an instrumental good.  That it's good-for something.   I'm noting that people commonly find the instrumental good, which is a moral bad, compelling.  For most of us...hopefully, slavery isn't the instrumental good being considered - but it doesn't really matter which real or perceived instrumental good is being weighed against it's own moral badness.

I am dealing with the here and now. Not ancient history.

Regards
DL