A lot of people are thinking about the mental disabilities that come with Down Syndrome but there are quite a few physical disabilities they deal with as well.
Disregarding the level of mental functionality they can achieve, they still have physical problems that can effect quality of life.
So if you inform expecting parents that they could have a child that will have vision and hearing problems, heart problems, digestive problems, neck and spinal problems, a weakened immune system, higher risk for leukemia in childhood and a shortened life expectancy, what would they choose to do?
Even if there were no mental disabilities associated, some would still consider not bringing a child with all of the struggles it would go through, into the world.
On the other hand, they can be quite hilarious, intentionally and not. One of my friends had a young man with downs in the family, so one day at work I got to hear the story of how they found him hiding in the closet with his dick in the vacuum.
As for spawn killing the downies, I guess it all depends on the parents and if they can or want to handle so much extra work. I know one lady who adopted a child with downs, he's her only child, he's a young man now and he's doing good. Even with the disabilities, he's probably costing society less than your average hoodrat spawn, who will only create more hoodrats to terrorize polite society with. Also, I never hear of downies shooting people, even in places like Chicago.