Well, you're wrong. Where is the line drawn for what constitutes a disease? Retardation? Autism? Eventually every child below, say, 130 IQ will be considered disabled and unable to find work.
Apply this to every other trait: cardiovascular health, strength, height, vision, etc. All forms of weakness can be considered a disease. The end product of eugenics is that mankind will be made into a docile and fragile monoculture.
>If you want make your baby smarter, taller, or more handsome, it's not so easy because these traits involve 1000s of genes.
And? it's obvious that the technology will eventually be capable of this, just not all at once. It starts with single-gene mutations, then it will be 10's of genes, and then hundreds and thousands.
That is the slippery slope: there is absolutely nothing about your reasoning that prevents one step from leading to another.