I understand the why, but it doesn't make him any less dead wrong. If you told people they were going to wake up the next morning with Down syndrome most of them would put a gun in their mouth. I certainly would, and not because I feared how society would treat me.
This article is just the latest incarnation of the "noble savage" myth; the worship of anything deemed "natural". Well guess what, worms that eat the eyes out of little children are natural too. So is malaria, and cancer, and blindness, and fibrodysplasia ossificans progressiva.
The "diversity of human existence" as he calls it has a greater variety of horror than ever imagined by the most bloodthirsty tyrant or tortured writer. We've spent centuries in a war to the knife against that horror and for someone to declare they're on the other side is a stupefying act of denial.