It's no less arbitrary than deciding that gypsies, who engage in petty crime and mistreat their children, forcing them to become beggars and thieves themselves, should not reproduce. If anything, Down kids probably bring more good to society than gypsy kids. And forced sterilization is arguably more ethical than forced abortion. Same things goes for alcoholics and drug users. Privately, most people agree with this position. The idea that eugenics is inherently bad is very unsound and doesn't withstand scrutiny.

Can't agree here. Regarding gypsies it boils down to the culture and rates of stunting, as evidenced by families who broke off the former and prevented the latter.

The way people with trisomia function in society is also a product of our nurturing culture. It's only recently, when such people started living longer lives thanks to advances in medical science, that their intellectual development gained more attention and it was revealed that they can actually be more independent than commonly believed.

That being said it all requires a huge amount of effort and if a person with trisomia has siblings, they're very likely to be deprived of attention. Additionally, if they're a first child, they're the only one due to this. That is what makes it a net negative.