What exactly is logically impossible about people with down syndrome being happier on average than those without it?

It could be true because their surrounding/family... would take care better of them than the average person, that I might understand, but still, it's really a stretch.

can you really say you're happy with something when you don't know what life without it looks like? You adapt. You make peace with it. That's human nature. Doesn't mean it's the best option.

It would break the feeling of superiority of people without that disability. Fact. So they can't believe it.