It's ignored because it's horrifically inhumane and also probably not even true. If you earnestly have this thought, I urge you to seek help from a psychiatrist.
I don't think he was saying it is good that "unfit" children die in mines, that would be a pretty horrific assertion.
I think he is just wondering why we don't consider the impact of immigration on genetic diversity. If we have nations that have huge immigrant populations and nations that have huge numbers migrating away from them it would have an interesting impact on genetic diversity. There are a range of traits that influence whether someone would migrate from a place or stay where they were born. I don't think pondering the effects of that requires a psychiatrist.
I think he is just wondering why we don't consider the impact of immigration on genetic diversity. If we have nations that have huge immigrant populations and nations that have huge numbers migrating away from them it would have an interesting impact on genetic diversity. There are a range of traits that influence whether someone would migrate from a place or stay where they were born. I don't think pondering the effects of that requires a psychiatrist.