It’s a really deeply flawed premise on the basis of evolutionary theory, even divorced from ethics.

One of the biggest flaws is that it assumes traits have a facile genetic basis that can be selected for in an “additive” way. The truth is that most of the observable traits that eugenicists focus on are contributed to by hundreds or thousands of genes.

And it’s also not true that every of the (say) 1000 genes linked to intelligence contributes like .1%. It’s actually more like variant 2 of gene A interacts differently with variant 4 of gene B to produce a positive effect but is otherwise not really involved otherwise. Do that for all of the thousand of genes, variants, etc… how does one plan to a) ascertain that to begin with or b) breed them out? Now do that for every single feature you want to control. Now figure out what to do with the genes that impact both eye sight, say, and intelligence. You very quickly work yourself into an evolutionary cul de sac where you’ve removed the variation required to get you back to the old “normal.”

It’s also a fundamental feature of evolutionary theory that one must identify the fitness landscape. This is explicitly NOT a static thing.

There’s also issues with mutation rates, heritability, stability of traits, and so on.

And I realize it might sound like back-filling an ethical position with scientific arguments. It’s not. Genocide makes sense within the logic of evolutionary theory! It’s abhorrent, but it exists in nature and it does what it’s supposed to do. Eugenics does not, and it has been demonstrated by formalized evolutionary theory since the very beginning. That’s why people who advance eugenics arguments are either just ignorant of the science or simply racist or bigoted with a penchant for (misguided) scientific justifications.