But my point is this: saying that polyamorous relationships "work out great if you are a high status male" implies that they invariably do not work out great if you are a female, and the Marsten family is a data point that refutes that (implied) part of the claim. It might be the case that polyamory generally works out better for high-status males than anyone else, but so what? Playing football generally works out better for men than for women too, but that's no reason not to let anyone who wants to try to play football.
Also, remember that, over the age of 30, women outnumber men. By the age of 50 or so, there's a clear majority. Older women would be better off if they shared mates.
Finally, remember that polyamory is not the same as polygamy. It could also benefit men who want more space in a relationship, or have had failed relationships because of this, since with polyamory your partner doesn't have to be upset that you don't spend so much time together, as they can spend time with their other partner(s).
Part of my argument is that in the right circumstances the partially sexually mediated drive for status in males turns violent. It seems like things like marriage and monogamy restrain this impulse and we all benefit from it.
Notice that I really don’t see marriage or monogamy as centrally enforced but rather as an emergent pattern of behavior that has been adopted by most societies and then codified into law. I presume it was this widely adopted because the individuals doing the marrying believed it was the best option at the time. Overall I would say that although the institution has to evolve to accomodate women’s control over their reproduction and the vast economic potential women have to offer, we are probably not so special and different now that we ought do away with it.
Regarding your point about aging, I actually think that should take a back seat to the wellbeing of children. It still seems to me like monogamy and marriage are the best vehicles we have today for raising them. Even if polyamory could potentially provide some benefits to the parents, I think those are the type of sacrifices that people gladly do for the well-being of their children.
The way I see it, the reason our society seems to be falling apart now is that women now have power and wealth, and aren't forced to settle for lifelong marriages to shitty, abusive men and being relegated to being full-time mothers whether they want that life path or not.