Is the childhood mortality really "naturally" as high as it was in the 1800s?

This is one thing I have always been skeptical of. Is the childhood mortality really that high (it was 50% in the 1800s) when you take away modern technology and modern medicine? Or was that just a transitory phase for humanity? You would think that with such a high mortality rate, they would adapt.

The defective children dying is itself an adaption.

Defective implies the absence of some genes that help fight diseases. If resistance to disease is heritable, we should see some families with high mortality rates and some with low ones, but my impression is in times when the mortality rate was high, it was high for all families.

It was higher for some, but you would get certain de novo mutations in any family.

The implication is that we are probably genetic shit, because if the natural course of things without modern medicine is that disease randomly kills 50% of the population every generation, then it would have probably killed us or our parents.

Yes, we mostly suck.