The infant mortality rate is generally describing babies dying in childbirth. Not thereafter, i.e. Bris.
And forget 3500 years. We’ve been practicing metzitza successfully for the past 50 and 100 years in “advanced” America. And in those 100 years, all the anti-religious have are less than a handful of uncoraborated stories from the last 5 years or so that they keep repeating. As if they keep saying the lie enough times it somehow becomes true.