I don't disagree with you. Unwanted children do not have happy outcomes, especially those with disabilities. However if you are applying the logic of adoption to that of eugenics you should tread carefully. If society is allowed to impose restrictions on adoptions, why not biological parents? Perhaps pregnant women should have an income test? If a mother can't afford to feed her child should she be allowed to retain custody or have the baby at all?

If a pregnant person decides they cannot afford it or aren't up for the commitment that's ethical. If a pregnant person decided to abort out of some pity for people with downs, well that's eugenics and ableist, so probably unethical. Pragmatically though the result is the same.