Down syndrome did not get better, our humanity with regards to them did. Like all people, when treated better, fair better.
Given the routine treatment of neglect and torture of people with Down syndrome throughout history and the relatively recent change in our treatment of them, we have very little understanding what someone with Down syndrome is truly capable of accomplishing.
During the same time as we have allowed the advancement of people with Down syndrome we have also created less invasive and cost effective means to identifying them prenatally through genetic testing fulfilling Reagan's hope toward ultimate prevention.
Down syndrome is a victim of a diluted and compromised narrative. The messengers with the most money and most concision are the medical and scientific communities supported in part by profitable genetic testing companies, all touting the same compelling message: people with Down syndrome are defective. Here humanity does not accept individuals with Down syndrome instead it sees them as a problem to solve.
The most vocal messengers who have a vested interest in preserving Down syndrome -- other than actual people with Down syndrome -- are parents who have children with Down syndrome. The dominant public narrative of parents is the reporting of their initial devastation at their child's diagnosis coupled with grief and finally acceptance of their child who has enhanced their over-all life experience.
The problem with this narrative from purely a human rights perspective is Down syndrome as acceptable shows up halfway through someone else's story. It was a message likely more potent closer to de-institutionalization when most children with Down syndrome were beginning to be raised in their family's homes rather than institutions.