First off, polygyny and harems aren’t necessarily the same thing. You also are neglecting to address that because polygyny did lead to partner scarcity, and the spread of STDs, for most of the past 10,000 years, patriarchy has meant enforced monogamy (at least for women), no matter how it first originated.
And since for the first 97% of human history before patriarchy people/women had sexual and social freedom, restriction of that, either in a monogamy or polygynous situation, was a significant decrease in personal freedom. Whether or not there was jealousy, women still had sexual and personal freedom until 10K years ago.
Patriarchy evolved for the main purpose of ensuring who the father is by restricting those freedoms (that’s where the term comes from, as you already know) so that land and animals could be passed to heirs of the blood. It’s the primary reason for the onset of patriarchy with the advent of agriculture, when people shifted from living in collective bands that shared everything, including child rearing, to having nuclear families.
“Women’s roles as community leaders shrunk. They were isolated in a new patriarchy, their bodies and babies mere chips in the agricultural economy.
From there, fierce competition for wealth divided communities into smaller units and eventually into nuclear families. Monogamy ruled, and still does.”
Blame agriculture for your basic sex life
Polyamory was the pleasant norm before humans learned to farm.
What Becky (and Jordan Peterson) talk about is reflective of ancient patriarchal models. Modern polyamory (and science) indicates that if there were no social stigma to it, women would also have access to as many mates as they wished (unlike ancient times), and it would not take us back into a situation where a small percentage of men have polygynous lives, with the vast majority of other men having no mates. It’s apples and oranges because we live in very different times.
Continuing to try to overlay ancient models over modern realities and monogamy-oriented paradigms (which tend to be based in patriarchal dominance hierarchy) overtop of polyamorous ones (which tend to be based in partnership models) is just going to further this place of essentially talking past each other because we are talking about very different things.
Edit: Newsflash — if you previously mated with whomever you wanted whenever you wanted and can now be stoned to death for not keeping sexual exclusivity, you life has not improved, even if you have more to eat on a regular basis.