Seriously, explain to me, how can anybody want to create a life that is objectively way at the lower end of quality of life?

"Objectively" does a lot of heavy lifting there. If you want to make the utilitarian argument, then make it, sure, but I don't think you'd find anyone advocating for not killing special needs people, but then turning around and agreeing that like, normal people should die, or suffer some dire fate in the stead of special needs people. I don't think we should really be pushing any orphans into the orphan crushing machine, personally, and I don't think it's probably an accurate dichotomy to say that the machine is inevitable.

If you also want arguments for why special needs people should be allowed to exist. People born without legs, they incur a certain cost on society, sure, but they also do a lot of good just by passively kind of existing. The ramps on the entrances of buildings, right, they're obviously for those people, but they can also be for elderly people who have a hard time with stairs, people who have lost their legs in some sort of incident, people who need to transport a large unwieldy piece of furniture. The ramps benefit everyone. If an intersection can be crossed properly by the blind, if it's designed for it, then, sure, it might not be the best idea, but you could cross it while on your phone, or reading a book, or generally distracted by whatever visual stimulus. And if we're doing all those things to accommodate people who aren't necessarily disabled, then it shouldn't matter that much whether someone is or isn't, because it doesn't cost us anything to just let them exist, and their insights can be valuable.

That doesn't even get in to how you might theoretically be able to, I dunno cure autism, or heart palpitations, or what have you, in the future, with gene therapy, making every life lost now kind of a short-sighted tragedy. Or how you could turn the logic around, and say, oh, well nobody really consents to being born, giving birth is unethical, like the psycho antinatalists do. Or how you could extend this logic to say, hey, maybe we should kill all old people, eliminate hospice care.