Killing off elderly people could potentially lead to eugenic effects downstream.

Can you illustrate how, in an example? You seem to be decisive this is true, but it isn’t obvious to me and I didn’t see an explanation yet.

If possible, try to link your explanation to concepts used in https://en.wikipedia.org/wiki/Eugenics, so that we don’t talk past each other.

From my possibly still uneducated point of view, what happens to elderly people (who don’t procreate) can not alter the gene pool.