The problem that anti-eugenecists have with eugenics is that eugenecism makes sense. You can only argue against it from an irrational, selfish perspective ("we have to remain, even if we are bad"). And the problem with eugenecists is that the end stage of eugenecism is essentially conceptual death.

A eugenecist wants the best people, the best society, the best relationship with nature, the best it could possibly be. They will tinker and tweak, until they reach that greatness; that harmony; that perfection.

The only issue is, when all the populations of the world become high-income societies that are then at or below replacement due to "universal education", none will be able to replace themselves without one group of people effectively dying off.

And even then, if everyone would be the same, would nations or "different" people remain? What would be the point? We'd truly, at that stage, have more need for a one-world government than a patchwork of nations. A eugenecist utopia, as it were. But that would be the death of the "world" as we know it. Conceptual death.

Even worse, if by some irrational "miracle" nations do remain, then people will, ironically, demand, and then force another people to become them, even though there is no difference between them. Tragically, this irrationality will just kill even more people as the various states (and the people within them) become desperate to survive by any means (return to the animal kingdom).

And, at the end of those wars, at that point, when the survivors start dying of old age, what is left?

The last person may then think that it's better that they kill themselves to "benefit nature" or something like that, rather than continue on as a parasite. Or, if AI achieves singularity, they may ironically choose to become "more than human", becoming something else entirely; a new species.

https://www.youtube.com/watch?v=JrBdYmStZJ4&ab_channel=BrianR