This starts from an incorrect premise — that everyone with Down syndrome “will perpetually need care and supervision” — and then heads downhill. “Misery” and “ends up in a hellhole” are choices society has often made in the past for people with intellectual disabilities, but they aren’t a law of physics or fundamental moral law.

What are the ethics (and societal obligation) of supporting someone who’s had a severe stroke? Or how about a traumatic brain injury from a car accident? Oxygen deprivation from near drowning? If these are different from a congenital condition like DS, why?

The same, which is why I support universal healthcare and expanding healthcare to include nursing support/housing for the disabled.

If someone gets cancer, then yeah they should be covered such that they aren't made homeless because of their disease.

If someone has a stroke that leaves them unable to work, again a social safety net that keeps them from being homeless should be in place.

The ethics are pretty simple. It's reasonable for a good society to support those in need through force of taxation. Just like it's good for a society to keep the water clean through force of taxation and regulation. Everyone benefits or has the potential to benefit from such a universal system that protects them from circumstances outside their control.