Doesn't follow. (TL;DR: use the social model of disability, Luke.) This only proves that either Williams syndrome is bad or society is screwed up.
You have legs (probably), but no wings, and you do fine in everyday life. Things often rest on the assumption you have functioning legs - stairs, narrow doors, curbs, floor coverings, and people with no legs have trouble using them.
Yet imagine a society where everything assumes that people have wings (but not necessarily legs). Most buildings have no doors, because of course you can fly through the window. Things are placed high above your head, because of course you can fly up. There are perches you can't use if you have legs.
You and I would do horrible in such a society, with exactly the same body configuration. Neither we nor the hypothetical legless winged person are "cases of a body gone wrong", we just live in societies optimised for body types that may or may not be ours.
Likewise, people with Williams syndrome may only need atypical caregivers and institutions because the type of care they need is abnormal in our society (unlike the type of care that consists of paying other people to make your food and bring it to stores near you). If almost everyone had Williams syndrome, maybe we would need supervisers.
I would rather have wings than legs. I think I might even rather this in our society.
Link: abcnews.go.com/Health/friendly-extreme-meet-kids-adults-williams-syndrome/