If you can abort a child on the basis of a pre-natal test showing the child probably has a serious physical or mental deformity, why not allow smothering the child after birth when you can be sure. Even better, why not wait a few years until you can be sure the deformity is all that bad (e.g. some children with Down’s Syndrome are able to go to school and hold down jobs – so why not wait a while) if the kid doesn’t measure up, dispose of him before he drags down the family.
I’m not sure what the Rabbanim would say. However some goyish legal systems have had provisions for getting rid of those whose lives aren’t worth living (if Ha-Shem slips up, maybe we can help by cleaning up the mistakes). The only real problem politically is that the country the led the way in “eugenics” and in supporting elimination of undesirable people from the population had to back track when they lost a war and their enemies decided that such behaviors were illegal and criminal (retroactively, of course).