I understand where they're coming from, but I believe it comes from a place of local and specific concern (the child with Down's syndrome) and not the wider impact.

The way I think about it; 10-20% of known pregnancies (and a larger number of all pregnancies) end in miscarriage, the majority of which are due to genetic errors and chromosomal abnormalities that, unfortunately, mean the fetus wasn't viable to begin with.

While some genetic defects don't kill the baby in the womb, the resulting baby is not healthy and will never be self-sufficient. Terminating these pregnancies lets the couple try again and gives the chance for another, healthy baby to come into the world, and possibly more because they won't have the burden of a many-orders-of-magnitude more difficult and perpetually child to raise.

It is a sound pragmatic logic (ignoring few corner cases), but people deciding things in such hard situation often don't decide purely on logic, if at all.