No disagreements there, but adaptations that are advantageous for the population as a whole won't necessarily spread through a population.
https://www.lesswrong.com/posts/QsMJQSFj7WfoTMNgW/the-traged...
Ah, I better see your point. Well, that adaptation has to be selected for at some point of course.
Among a single species, with a 50/50 split of a genetically diverse and homogeneous population, if you eliminate 10% of the former and 75% of the latter, the split becomes 80/20. If that distinction stemmed from genetic factors, you have selected for diversity.
Genetic diversity isn't only advantageous to the population. It's advantageous for the individuals that carry it, depending on the circumstances. Therefore it can easily be selected for.
The less diverse the population, the less likely timely mutations will help protect it. Incidentally, our modern society is pretty diverse, especially as we protect those who couldn't otherwise survive without modern medicine.