"Unlike the pronatalists, who want to solve depopulation problems by bringing in entire new demographic cohorts of people, who are likely to have radically different values from us, on a mass scale, I'd rather we preserve our culture and focus on keeping alive and healthy the people we already have."

Add enough scale and time, and cultural preservation disappears.

But as a side note related to this, I'll say it right now: effective altruism is 100% an exploitation scam that seeks to use hypothetical future justifications for executing actual in-the-moment exploitation.