Developers usually make $50-300/hour.
That seems like an overestimate even for US. More importantly, I don’t think most open source developers earn this much money (otherwise they wouldn’t ask for tiny donations), and hence it’s not the relevant figure. If I’m wrong about this, please do tell me - I very much would like to know if the hours I occasionally spend on open-source contributions can instead earn me hundreds of dollars. ;)
Sure, it’s possible for a person with a severe disability to grow up happy. But when one is making a decision in real life (like having a child), one should consider an average case, not a exceptional one. And the average case for an example like Down’s Syndrome is pretty bad. It is a bit unclear how to quantify the suffering in this particular disease’s case because the main harm to the child is lifelong mental impairment and assorted physical disabilities - but it is at least going to inflict suffering on the child’s family, since caring for a child with a severe disability for their entire life isn’t exactly fun.
I don’t see the relation. You’ll notice that I’m not proposing killing off disabled people for the “improvement of society” or whatever it was that nazis called it. I am not doing this because nonconsensually killing a person is a harm to them. But deciding not to have a child isn’t the same thing as murdering a person - it’s not harming anyone who exists, and hence may well be morally better than having a child.
(Oh, I suppose you might mean that I’m arguing that there are circumstances in which it’s morally bad for a person to have a child, which is similar to nazi eugenics in that I’m deciding whether or not people should have children? In that case, my answer is that the difference is that I’m a person, not an authoritarian government, and I don’t have power (nor, indeed, the desire) to force people to obey my personal moral judgements.)