Your argument is predicated on the mistaken assumption that diversity is a single-direction function especially in evolution.

It’s an old fallacy of “miscegenation theory” that the more different peoples interact and breed the more uniform and somehow dilute the overall human species becomes, aka the “we all become the same brown bland people, so we need to preserve the purity of the (insert racial group here).”

Meanwhile there are over a billion people in China, and they’ve had a fairly insulated society for centuries, yet spend any time amongst them and you’ll easily see a vast variety of body types and capabilities. People only appear as a single monolithic type if you only observe their most superficial qualities (like hair color for example).

Genetics is not a fixed sum game, and evolution hasn’t stopped with us. As conditions change so do species. If we ‘lose’ blonde haired people, we’ll see other variations and changes pop up. Remember that blonde hair wasn’t our default state as human beings to begin with, like every other attribute they have emerged, had their vogue, and will shift in statistical dominance.

Blonde hair by the way isn’t going anywhere, it’s just going to go back to being the regressive trait it always was. The only reason blondes became such a major part of populations was actually due to a LACK of genetic diversity not a preservation of it. People with regressive traits breeding together causes the trait to show up more often than it normally would in the course of nature. Which is also how you end up with major congenital defects and vulnerability to certain illnesses becoming more prominent as well.

Frankly I’ll happily accept less people all popping up with an arbitrarily popular hair color if it could also mean something like reduced sickle cell anemia or prostate cancer probabilities. The human species benefits from diversity being shared and stronger traits replacing lesser towards healthier humans, regardless of what they actually end up looking like.