There are still many people who do not do Dor Yeshorim, which is why diseases such as Tay-Sachs still affect our community.
For those who were wondering why our parents and grandparents didn’t have children with these diseases, like Wolf and oomis said, there’s no doubt that children did die from them – they were simply unheard of. Cancer, MS, Lou Gehrig’s disease, even Down syndrome and other conditions EXISTED before they were “discovered,” but people were not diagnosed or treated for it. People with diseases and conditions that are well-known and often treatable today died from their undiagnosed conditions, were shunned, or simply deemed “meshuga” years ago.