You have it backwards here, Down syndrome is unusual for being viable not because the symptoms are so terrible. Most chromosomal anomalies result in a non viable egg / sperm which doesn’t result in a live birth. The majority don’t even last long enough for someone to notice they were potentially pregnant.

Turner syndrome is another example without the mental issues: https://en.wikipedia.org/wiki/Turner_syndrome

In the larger context, there may be some long term advantages to chromosomal anomalies because their number varies so much between closely related species.