Even if strange when it comes to animals, humanity progresses following rules similar to classic evolution, aka by randomly mixing stuff until something works. We've gotten (only) pretty good at selecting and amplifying what works extremely well, but we still need a steady stream of randomly arranged characteristics to enter the pool. Imagine if the next Newton isn't born (or is born 200 years later) because a couple decided to not have children to "save the planet". Perhaps this figure would've been a key piece in a breakthrough discovery about energy, climate, terraforming, public policy....
I seriously fail to understand anti-natalists.