Remember, too, that not only was Hagar a slave woman, and not only was Abraham's getting Ishmael on her an act of doubt of God's promise of posterity out of Sarah; but also Hagar was in the first place a woman of Egypt, not a Jewish woman: Jews were not allowed to buy other Jews as slaves.
Of course this was before the first five books of the Old Testament were written. But it seems to have been the custom already.
If Haggar had gotten a good lawyer, better child support, and a binding financial palimony or surrogate package- with benefits - and a condo in Dubai where Sarah didn’t have to see her every day - history might have turned out different.