I am coming at this from an angle of not wanting to have any kid at all. To relate it to the article - "To have a child is to begin a relationship that you cannot sever. It is supposed to be unconditional" - I am not really fine with that kind of responsibility. And kids are a lot of work, work I don't think I'd enjoy at all. With choosing to have a child with Down Syndrome it seems like you are choosing that work for the rest of your life.
This discussion has been happening in Sweden too, with some saying that screening for trisomy 21 is immoral and bad for society. I viewed that as kind of selfish in the past. People who had the means and will to take care of a child who meant so much more work wanting to force that on others for some kind of benefit to society as a whole. And I also viewed it as dishonest when they talked about their kid who was happy all the time and so so sweet without mentioning any of the undeniably negative effects on health Downs Syndrome can include. I viewed it as people wanting to deprive women of a choice in order to further their own agenda.
Now I have a bit more of a nuanced view of it, and recognize that it isn't really that simple. Women (and families) are offered a choice but maybe not a fully informed one. Scanning for trisomy 21 doesn't necessarily give all the information that we often think it does. And I understand that parents of children with Downs Syndrome who don't want it to be a routine screening dislike it because they think it gives prospective parents an uninformed choice based on preconception and not facts.
And I agree that society's view of success and a good life maybe isn't really the best life one could have. And that the values we hold maybe aren't always the best. Your human value shouldn't be in the work you can preform or the accomplishments you can have.
Like, should the answer to people with disabilities facing difficulty be to make society more supportive or to have people with disabilities not be born? To me the former feels more humane. But at the same time you can't really just ignore the fact that some disabilities lead to a worse quality of life. A disability causing constant pain or leading to an early death isn't just "a different way of being human".