This is a pretty inaccurate exaggeration. While death in childhood was a lot more common it was more like 30% in the middle ages and lower then that by the 1800s. And while people living to 80 wasn't rare it was by no means as common as today given that diseases and injury were both more common and treatment was worse. Childbirth was also a very dangerous time for the mother and accounted for a substantial amount of post-childhood mortality.
Yes I agree in the old days childbirth was more dangerous for women than warfare was for men (at least in that warfare isn't usually constant but pregnancy was).
I'd disagree in that the fundamental point is under those circumstances people would be dying of diseases and such faster than they can die of starvation.
As a concrete example where I live the civil engineers provide wells for water and sewage treatment plants dumping safe waste into the river. Without that engineering, people would die from waterborne illnesses faster than they can eat canned foods.
Sorry, my comment isn't about how people would die post-apocalypse, it was about you saying that 80-90 percent of people died before age five before the year 1900 which is much higher than reality (probably about 30% in the middle ages, less then that in the 1800s) and also your assertion that if they survived to adulthood they lived to 80 at a similar rate as today when in fact it was much less common.
