It's also possible that genetic disease may occur even though the parents were from good lines.
Of course it is possible, but it decreases the probability.
It's just unreliable.
I think you believe it is unreliable because you don’t want it to be reliable. Perhaps there is something about the idea that makes you uncomfortable. You can criticize the idea as much as you want, but I don’t think there has ever been an attempt to create a large scale eugenics movement. And I am talking about real eugenics, not the silly experiments that were performed by the Nazis. A eugenics movement in the 21st century would involve various things like gene therapy, germ-line genetic engineering, selective breeding, and cultural change. I wrote a long post about the things that I would like to see during a eugenics movement. My ideas are not perfect, but they don’t involve any unethical things like forced abortions or sterilizations.