I think if parents don't feel they have the capability to care for a Down's syndrome child, perhaps because of difficult family circumstances, it should be their choice.
However, to assert that parents have a "moral duty" to abort such a child is, I think, totally wrong. I can see that there are concerns as to what will happen when parents die or become ill or disabled. But my feeling is that a decent society, especially one with our resources, should provide early and ongoing support to such children and their families. Then, in the event that a parent reaches the point where they can no longer manage, there should be be a smooth transition to good quality - and caring - supported housing or a residential home.
My husband worked with people with learning disabilities and I often went to social events in various residential homes in the borough. It seemed to me that those with Down's syndrome and other learning disabilities led, on the whole, happy and fulfilled lives.