Sorry. Doesn't work that way. God Himself instructed Hosea to marry Gomer, a prostitute.
Per your logic, Gomer would have given up her prostitute role post-marriage. She didn't. (She made it part of her "cottage industry").
Jacob didn't launch into polygamy because God told him to...Deception was the root...and then wives' child-bearing race kicked in re: them having their servants sleep with him. Deception...jealousy...hardly "godly" motivators.
Nobody ever referenced Hagar as a "wife" except Sarah. And then only once. After Abraham slept with her (for all we know, it may have happened only one time), the Angel of the Lord doesn't reference her as a wife...only a servant of her mistress (Sarah).
Abraham doesn't.
Moses, who compiled Genesis, doesn't several chapters later.
The apostle Paul, when mentioning Hagar in Galatians 4, doesn't.
She was still a mere servant-girl status post becoming pregnant.
I believe she did the same things that a wife would do and since she was given to him by Sarah she was a wife, what ever we wish to call it she was given to him by some one who had that right.
Jacob had two wifes and two concubines, the father had the right to give away his daughters and his daughters had a right to give him their servants, and all four did the things a wife would do.
You say God had nothing to do with it which may be right but that does not change the fact that Abraham and Jacob are the Patriarchs of what has become the Christian faith, i do not think they would have been if God had counted Polygamy as an abomination.