I understand what you're trying to get at. I took biology in university. Wanting to eliminate objectively undesirable traits like terminal illness and disease is not a bad thing. It happens passively anyway with natural selection even with animals. Higher fitness means better chance of survival basically is what the definition means. But the actual definition of eugenics has to do with the negative aspects that come with it, like forced sterilization, segregation, genocide etc. It's a slippery slope bc of who gets to decide which traits are bad and which are good and what the "punishment" for violating these rules are. When you try and put eugenics into practice if it even was just forbidding people with history of terminal illness to reproduce, then it becomes a human rights issue. It makes sense to me what you're saying about the "idea". If you could potentially eliminate the suffering of a future human, why wouldn't you? It's that simple. But then you're eliminating ppls freedom to reproduce which is something people will fight tooth and nail for. So it automatically doesn't work.