The problem with "eugenics" has never been the science (the science science, not the race science); selective breeding works with every other species in the world, it'd work with humans too. The problem is that out of every other species in the world, humans are the only ones to have developed "being systematically horrible to each other based off totally meaningless bullshit" and we've kinda embraced that as our whole thing.

In a perfect world where we don't have to worry about any of the practicalities, eugenics could be a wonderful tool for improving humanity as a whole. But humanity is too intrinsically broken to be able to handle it, and the practicalities we're glossing over in this case pretty much instantly become "forcibly sterilizing people we don't like" every time someone brings the idea up.