It works if it weren't unethical doesn't make the argument you think it makes.
The notion that we suck at choosing the good genes is entirely misled, even if it is just sarcasm. The final question is also morally misled because science and the notion of truth is not amoral. Science, without humans, doesn't exist. And humans are moral beings (constrained by social and moral considerations).
Eugenics is one such field which notions cannot be true because its axioms are inherently unethical. "It works" is not an isolated amoral argument. If it needs the morals of a society to be radically altered to work, then it is not science. It is just racism in a lab coat. The case of dog breeds, for example, doesn't support eugenics. On the contrary it dispproves it.
We have genetically altered dogs (and many other animals) by selective breeding in ways that, according to eugenics, should've eliminated inbreeding and genetic defects. Guess what? it hasn't done that and actually might have made it worse. Historical analysis lead us to the idea that running wild with eugenics will always lead to genocide, regardless of which genetic traits are selected as the best, eugenics is genocide. So, it cannot be severed from its ethical considerations. Science cannot exist devoid of ethics.