People have a really rosy idea of an ideal polygamy. I cannot think of any purely happy polygamous marriages in Tanach which didn’t contain some kind of angst due to the fact that it was polygamous (Avraham, Sarah and Hagar; Yaakov, Rachel, Leah, Bilha, Zilpa; Shlomo and his thousand wives; Elkana, Chana and Penina; the Pilegesh beGivah; etc.). The only person who is legitimately told that he can marry multiple wives- with strict regulations- is a melech Yisrael, which is possibly why Dovid’s marriage problems did not relate to his polygamy, IIRC. The prototypical marriage mentioned in halacha and in Tanach is between one man and one woman. Just because something is muttar doesn’t mean that it is a good idea. Besides, IIRC (and I could be wrong) in the time of R Gershom, the issue wasn’t with actual polygamy but with men not needing to divorce their wives in order to remarry. That’s not something we want happening again (even if we do have the heter me’ah rabbanim…).
Hey, you know what? Heter me’ah rabbanim! Put your theory to the test. Find a couple who would want a “sister wife,” find an older single who would want to marry the guy, and find 100 rabbis to sign agreement and you don’t need a whole big patchke of rescinding the whole thing.