Not everybody who makes it to adulthood has kids. Various disabled people don't find a partner with whom to have kids.
I'm relatively confident that medical progress will be stronger than the effects you talk about in the next century. Sooner or later I expect genetic engineering to replace natural selection as the driving factor for human DNA, so the effects of the selection become less important.
If it's worth saying, but not worth its own post, then it goes here.
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