But even giving them less then optimal features - intelligence, strength, looks - is quite equivalent to making them stupidier, weaker, uglier.
I don't believe that killing someone is equivalent to letting him die. Why should I believe that making someone stupid is equivalent to letting him be stupid?
Also, cheating on someone to improve the health of the offspring results in a non-identity problem since the offspring is not the same one that would have been created without cheating, so whether the offspring is benefited is questionable.
You're right. I got way too far with claiming equivalence.
As for non-identity problem - I have trouble answering it. I don't want to defend my idea, but I can think of an example when one brings up non-identity and comes to wrong conclusion: Drinking alcohol while pregnant can cause a fetus to develop a brain damage. But such grave brain damage means this baby is not the same one, that would be created, if his mother didn't drink. So it is questionable that the baby would benefit from its mother abstinence.
Like any educated denizen of the 21st century, you may have heard of World War II. You may remember that Hitler and the Nazis planned to carry forward a romanticized process of evolution, to breed a new master race, supermen, stronger and smarter than anything that had existed before.
Actually this is a common misconception. Hitler believed that the Aryan superman had previously existed—the Nordic stereotype, the blond blue-eyed beast of prey—but had been polluted by mingling with impure races. There had been a racial Fall from Grace.
It says something about the degree to which the concept of progress permeates Western civilization, that the one is told about Nazi eugenics and hears "They tried to breed a superhuman." You, dear reader—if you failed hard enough to endorse coercive eugenics, you would try to create a superhuman. Because you locate your ideals in your future, not in your past. Because you are creative. The thought of breeding back to some Nordic archetype from a thousand years earlier would not even occur to you as a possibility—what, just the Vikings? That's all? If you failed hard enough to kill, you would damn well try to reach heights never before reached, or what a waste it would all be, eh? Well, that's one reason you're not a Nazi, dear reader.
It says something about how difficult it is for the relatively healthy to envision themselves in the shoes of the relatively sick, that we are told of the Nazis, and distort the tale to make them defective transhumanists.
It's the Communists who were the defective transhumanists. "New Soviet Man" and all that. The Nazis were quite definitely the bioconservatives of the tale.