It wasn’t just child mortality. An 18 year old was only expected to make it to 66. Life expectancy for a 40 year old man was just 29 years, meaning they wouldn’t see 70. Today a man who makes it to 40 should reach 77. A 65 year old past 82.
Not just that, but the fertility rate declined bigly. Basically the left created the SS Ponzi scheme where the math depended on the then high fertility rate to create new "investors" (younger workers) to keep making payouts (SS checks) that were higher than the older workers put in themselves (especially the first generation of SS recipients). But after the left created the Ponzi scheme, the left heavily promoted for western societies to lower their birth rates to save us all from the overpopulation armageddon. So now that the birth rate in the U.S. is 2.1, the SS Ponzi scheme doesn't have a chance of money paid into SS keeping up with the obligation to SS recipients.