The Cherem was established because the goyim were persecuting the Yidden since the goyim were jealous Yidden could have more than one wife, while the goyim’s religion prevented them from doing so. (Limiting to one wife is inheritantly a Christian value, not a Jewish one.)
Which is also why the Cherem was only accepted by Yidden in Christian lands, while the Sephardim and other non-Ashkenazim (i.e. Teimanim) never accepted the Cherem and continued to be allowed to have multiple wives.
Now that Christian persecution of Jews is B”H mostly a thing of the past, I don’t see why we couldn’t do so again today. (BTW, the Fundamentalist Mormons openly practice polygamy in America today. They even have a tv show about one such family. Mitt Romney is a regular Mormon, and his great grandparents were polygamists.)