Firstly, I have to correct your term before I can answer it. The correct word is P-O-L-Y-G-Y-N-Y (not a typo!). Not Polygamy. Polygyny appears in The Hebrew Bible, not Polygamy.
Just as many sins were recorded, P-O-L-Y-G-Y-N-Y is not an exception. It was a sin described, not a way commanded to live.
It was never Abraham's idea to have a concubine, it was Sarah's lack of faith. Rebekah was wiser and instead of a concubine, she and Isaac PRAYED to get pregnant. Imagine that! Prayer instead!
Your idea of valuing women on a scale to be wife or concubine is narcissistic. The same could be reversed on men: "a man had to be in his prime, able to hold an erection, have an 8 pack and be rich or he would never get a wife. The old men would be discarded at 50 when their ability to hold a stiff erection for long periods of time diminished". See how I can create made up narcissistic narratives to dehumanize men?
Yet the truth is that throughout history people found spouses compatible to themselves, regardless of their circumstances. Widow women found widow men. Single mothers found single fathers. Virgin men find virgin women. Poor men go with poor women. Divorced men choose divorced women. It's not about a heirarchy, it's about compatibility.
There's nothing wrong with a man picking a wife solely because of her looks (as long as he tells her upfront that's the reason and foundation of the marriage) and there's nothing wrong with a girl choosing a guy for money (as long as she disclosed it upfront), but that doesn't mean every guy and every gal follows that pattern. People pick life partners based on many other reasons besides money and sexuality. That's been all throughput history.
Either way, Polygyny was always a sin and always will be a sin
In this paper the scriptural basis for Monogamy is discussed ©2013, 2016, 2019
www.academia.edu
You'll find in my essay how the evil men are recorded to value barren women above those that have birth. Interesting. Nothing new under the sun!