Somehow, the cold statistics of DNA mutation would leave us behind, and we'd become creatures with no means of adapting, until one vast catastrophe forces the issue.
What might feel practical in the moment, for one, might be terrible for the collective.
But he wasn't talking about that. A lot (most?) people with serious disorders will not have children, and from an evolutionary perspective that's practically the same as if they died young. There's already selection against those traits.
Following that line of thinking leads to notions about the presence & importance of second-order interactions (especially the impact of ideas on the survivability of a group); these make it hard to reject the hypothesis: production of outlier individuals is essential to our species.
Sadly outliers occur on all sides of the distribution, including the unhappy ones.
Things might have turned out much differently for these children had they been treated differently while growing up.