If it was due to reduced infant mortality then why didn’t the same pattern happen from 1800 to 1880? Infant mortality dropped from about 1/3rd to 1/5th dying before their 5th birthday. Fertility rate being fairly consistent around 5 with a slight (10%) increase for a few years around 1815 before going back to around 5.

Then fertility rates nosedive from 1880 to 1935 with a temporary increase (boomers are the peak of this) before dropping down again and continuing to where we are today.

I get it, people have been treated badly throughout history and it even continues today. But that isn’t the reason the boomers exist.