.
Poorer folk back then would have only been able to marry one or two wives at most anyways, and we see this in the modern Muslim world, as most practiced polygamy is usually within a familial context, cousin marriage, etc. The fact that is, is that its exceptionally rare without some form of wealth or advantage for a man to have more than two to three wives in a polygamist marriage.
Where as Monogamy which is far from its own issues, does have the added benefit that in an ideal world the partners should be more committed to each other. There should be stronger emphasis on the raising of that group of offspring, and a closer bond between the parents. Which is something you don't get in a polygamist setting. You ultimately have to choose a favorite wife, a favorite group of children, etc.
No, poorer men would generally not be able to marry, because there would be a lack of women to marry.
The question becomes what to do with the surplus men. Islamic countries sent them to war and to do raids to get among other things, female slaves.
Now they'd probably live isolated playing games and watch porn.
In china, they do not have polygamy, but they do have 135 men per 100 women. As a result, crime, traficking and prostitution soar.
Polyamory then is socially a poor choice culturally for a country, unless you have a war to send them to.
Of course the sjws that typically champion polyamory are gender denialists to a degree and would ask why you wouldn't have women in combat roles or why polyamory would only have to be one male - multiple women.
In practise, polyamorists are more likely to have lowered barriers for whom they have sex with or develop ties with. They profess to exchange jealousy for compersion, a supposed joy of seeing their partner sexually fulfilled by another.
And in the polyamory scene you see a lot of women who have frequent sex with a smaller number of higher status men and a group of men who rarely or never have sex.
Of course the lowered barrier for sex means they are likely to get thrown some sex, out of pity, out of convenience.
A considerable amount of these people are on antidepressants or will end up killing themselves. I'm sure it works out well from some people. But having organized events for various polyamory activists, they put on a much brighter facade for the outside world. Even in the short time I saw some of these famous or semi famous activists, particularly how they treated their supposed loved ones, has soured my ability to look at polyamory as anything other than a big dream doomed to fail, an oasis that turns out to be a mirage.