Avira > Academics say that the cherem was because goyim had forbade it. Rabbeinu Gershom makes no mention of that. He says it’s because people can’t do it for the right reasons.
I understand your instinctive reaction at goyishe chochma, but we are all social beings. And rejecting history is apikoiresus. Gemora is full of examples of how institutions developed over time:
schools – parents -> Yerushalaim -> big high-schools -> small local schools;
ketuba – prepaid sum -> in F-I-L house -> mortgage on property
same thing here: people not doing for right reasons may correspond to the fact that non-Jews were not doing it any more. Polygamy was not very popular (R Stensaltz writes that he knows of one Amora or Tanna who had 2 wives), so these will be strange people under strange circumstances who
will use it. Given the surrounding culture, people will be even less willing to do it, so the remaining cases will be even “weirder”.
Also, note that Sephardim who lived in a culture permitting it, somehow were able to continue having this option. Did they have better attitudes?