You are forgetting that reproduction in a human society is not simply about having the best genes, but having the best environment to raise your kids in. If 10 women all have babies with the same man, who is going to provide, help feed, clothe, teach, raise their kids? They either have to do it all by themselves, or they have to find another man who will do this for them while neglecting his own biological prerogative to reproduce, or they have to rely on a state apparatus that steals from men who don't reproduce, in order to give to the babies of those men that do.

In either of all these examples, the quality of the childraising will be much poorer than in a stable, traditional, nuclear family structure. If the woman has to provide for her children all by herself, she is much more likely to be poor and to not have the time to properly teach and raise her children. If she relies on a beta male to help do this but not reproduce, he's more likely to be resentful and mistreat the children, or simply be incompetent and do a poor job at raising them. If she relies on the State... Well, the State wants something in return, and what they want is to turn her children into state worshipping zombies.

Furthermore, the genes that are "best" under a situation of widespread monogamy, are different to those that are best under a situation of widespread polygamy. For example, a woman who only sees men as a sperm bank, may not be interested in the man's capacity for love, bonding, teaching, caring, protecting, or providing. She will simply look for biological markers of sexual attraction, which, interestingly enough, may not even signal good health or chance of survival.

Finally, you are forgetting about what happens to all of the men who can't find a mate in your polygamous society. They will be much more likely to be violently aggressive, to generate chaos in general.They simply have much less to lose than a man who has a stable mate that is bonded to him for life. This also affects women, who will feel lonely and in despair when they're all alone with their children and no man wants to stay with them for more than a night or two of mating, and this will impair the woman's ability to nurture and care for her children, also. But the effect is clearly much worse for males, which also have a greater capacity for harming others in the society, and not just their own families.

A society such as this is much worse for everybody, for the women, for the men at the top, for those at the bottom, and especially for the children and future generations. It's not an accident that, like I mentioned in my previous post, all societies that became prosperous did so by introducing monogamy and nuclear family structures, and the extent to which they became prosperous is directly proportional to the extent to which the society was able to achieve a widespread and faithful implementation of this model.