When you look at communism, you can just look at the result (failed economies, empty shelves) to see how bad an idea it was.
When we look at the arguments against eugenics, they are always about how it was done, not about any results. People were sterilized or killed based on some perceived abnormality or inferiority, it was extremely crude and inhuman, I completely agree. But all those arguments disappear if we do things in a more modern way while respecting human dignity, using today's technology. Unlike communism, where it's hard to imagine a method that might actually work.
If you understand anything about evolution, it is quite clear that the human genome will tend to degenerate thanks to the comfort of modern civilisation. I'm not talking about things like intelligence (because apparently that's one area where evolution is sill alive and kicking, intelligent people being more attractive contrary to what has often been feared), but look at our senses (sight, smell, hearing), physical strength and endurance, resistance to diseases, etcetera. Our genome gets bombarded with random changes all the time, and natural selection isn't weeding out the bad changes (which are the vast majority of all mutations). We do all we can to help people overcome their defects, lead a normal life, and reproduce. Which is great, but it does mean that in the long term we will need some other mechanism to replace natural evolution. Because evolution is not just necessary to evolve a better lifeform, it's also necessary to keep it from degenerating.
Moles evolved from some kind of rat that probably had very good eyes, yet after thousands of generations moles are as good as blind because nearly blind moles have exactly the same chance of reproducing as moles with better eyes. The same is happening to us. Old-style eugenics would simply say "ok, let's sterilize all people with bad eyes, then". Today we can just say "hey, we found the cause of your bad eyesight, it's a mutation in this gene, and we can make sure your children don't inherit it". Or even replace the bad genes with a better version. What's wrong with that?
If we don't do this at some point, we will constantly need to improve all our medical techniques to fix all the defects people have been inheriting. Babies will hardly ever be born without needing immediate medical attention after birth, because we can't allow mother nature to kill the bad genes.
Wouldn't it be better to just start the next phase of evolution by taking things into our own hands? Natural selection, which is cruel and indiscriminate, has all but disappeared for us, and that's a good thing because we can do much better than that now. But we'll have to actually do that some time.