Infanticide vs Natural Selection

Consider the following conversation:

A: The Spartans practiced infanticide and were therefore immoral.

B: "I don't support infanticide, but the Spartans were hardly alone. Moreover, before the advent of modern medical technology, caring for the "unfit" could be quite a challenge. One could even argue that infanticide filled the role once played by natural selection. Today, there are many people who couldn't survive without modern medical technology and many pass on bad genes to younger generations. Again, I'm not advocating infanticide. I'm just stating an unpleasant truth."

A: "It's intersting that the spartans did't kill infants, they left them out to die of exposure. Killing them would have angered the gods, so the spartans left it up to them if the child died. This kind of tinkering with natural selection is in the long term, foolish. Diversity is insurance against failure."

B: "The Spartans were far more advanced than our distant ancestors and were fully capable of nurturing infants with severe disabilities. The situation is more extreme today, with countless people who couldn't survive without modern medical technology. I don't advocate killing the "unfit" to improve the gene pool, but arguing that we need the unfit to survive is a little strange. It certainly doesn't work that way in Nature.

A: "Interesting point of view. Killing kids at birth gives them far less chance of surviving naturally than not killing them at birth. "

What kind of fallacy is this? At first I thought STRAWMAN. However, on closer inspection it looks like there's something going on - almost a kind of paradox.

There is a lot to unpack here. I see "strawmen" from both A and B... a strange conversation where it appears neither understand what the other is saying. Also, neither are being clear about what they mean to say (or I am just missing their points).

Yup, Strawman it is for the finish!

A: The Spartans practiced infanticide and were therefore immoral.

(Petitio Principii: Begging the Question). This assumes that infanticide is immoral, but based on what system of mores? Obviously, the Spartans held a very similar moral system to many other cultures, ethnicities, and religions of that period and even a few later ones, i.e.: Mayan, Jewish, Canaanite, etc., all of which practiced child sacrifice as a supremely moral religious act. SO we cannot immediately assume it is immoral on its face without erring into the fallacy of chronological snobbery, or special pleading.

A: "It's intersting (sic) that the spartans did't (sic) kill infants, they left them out to die of exposure. Killing them would have angered the gods, so the spartans (sic) left it up to them if the child died. This kind of tinkering with natural selection is in the long term, foolish. Diversity is insurance against failure."

The claimant appears to contradict their initial claim by drawing a Weak Analogy between immorality and natural selection. It might be a valid point otherwise but now seems to be deceptively moving the goal post from a moral argument to one of science.

The problem with leaving the weak to die is that in our culture it is not just physical prowess that makes a contribution. Imagine if Stephen Hawking had been left out to die because he was physically weaker than other children. He still made a contribution regardless of his disabilities, even considering whatever drain on resources it required to keep him alive.