No, it isn’t. “I wish you didn’t exist because your existence inconveniences me,” is a step away from “I should be able to kill you because you inconvenience me.”
You will likely think that I am being hyperbolic (“We’re just talking about if it was a good idea for these people to exist, not saying we should be able to kill them now that they do exist.”), but I suspect that you would not feel the same way if it was your existence being discussed this way.
Oh wait, no, that didn't happen, because in both your and my case it turns out humans are able to distinguish between life that is and life that might be. The "step" you mentioned is only a small one in a philosophical sense, enormous otherwise to the point of not being a concern.
They’re trying.
“Besides” is doing a lot of work here.
Socializing the costs this way has its own ethical problems, especially where the parents continue to reproduce after learning they are carriers; I’ve simply concluded that the costs of care are completely negligible when you contrast them with the loss of human dignity that results from valuing an individual human on the basis of economic cost or contribution.
Accommodating for a human that exists, if suffering, is clearly a moral obligation. Doing so when it’s not a human but a husk still is not, and deciding in favour of the very human parents—who also have a right to happiness—is definitely ethically valid.
In a related discussion, someone argued that keeping up industrial farming is just, because if we stopped doing so all the cattle that wouldn’t been born would be worse off for never being alive, even if their existence was suffering, because suffering is better than not being at all. I firmly believe this is just wrong. Before a being gains consciousness, it’s not a being and doesn’t experience, hence by avoiding their conception we also avoid unnecessary suffering.
You bring up an interesting argument, but I think there is some nuance here. I am not arguing that we have an obligation to propagate human life for the sake of propagating human life; I just think there is a risk of devaluing existing human life by claiming it ought not to have existed in the first place.
There are limitations here. e.g. if one is offended at the claim that Down Syndrome is something to be cured, it may be that one is placing too much emphasis on identifying the expression of an individual’s genes with the individual himself (so e.g. eliminating the extra chromosome is not analogous to eliminating the person himself). We wouldn’t do this with a broken bone, but the solution to a broken bone is setting the bone, whereas the “solution” to Down Syndrome has historically been abortion.
I can’t speak for the parent commenter of course, but this is by no means a universally accepted truth.