I had a uncle with Down syndrome. He was the sweetest and funniest person, we remember him every day more than 10 years after he passed away. Down syndrome carries a lot of physical health problems like heart or lung diseases which make their life very painful. He suffered from lung problems since he was 18 until he passed away at 49, living in a lot of pain and being a big burden to my mum and my grandma, who took care of him. Still, it's true, he never lost his smile and love her sister and mother back as much as it's possible, giving all of us who lived with him a lot of joy.
I am very conflicted with these kind of issues, but I think I am of the opinion that it's better to prevent this suffering, but once they're already here we should make their life as easier as possible.
I chose to call it quality of life because I don't think that simply being happy is enough to have quality of life, but I don't agree that it's about valuing intelligence over happiness.
It's a condition they, and their family, have to live with their entire life. You can't really be permanently sad about a condition you have literally been born with and can't expect to change.
Meanwhile, there are conditions that significantly decrease quality of life even though one's intelligence is unaffected. I think the factor is better described as choice. There are a large number of things a person with Downs just does not have the choice to do differently.
I know that I'm in the small minority of people that read Flowers for Algernon and didn't think the ending was a sad one. His life was interrupted with some brief magic and resolved into what it was always meant to be.
People have gotten emotional with me about my take on that, and that's just fiction. I guess my point is I don't think there is a clear morality play here. This is more like a trolley problem where you have to decide for yourself how much control you're comfortable with.
I am very conflicted with these kind of issues, but I think I am of the opinion that it's better to prevent this suffering, but once they're already here we should make their life as easier as possible.