I do have to say, I've never understood the European/Western liberal (in the broad classical sense, not the "social democratic sense", but more strongly among social democrats) impulse to devalue one's own culture and values so very much that one would rather go extinct in the process of helping others than survive in any form.

Yes, our planet cannot currently sustain a population of 9 billion (projected population peak in 2050) living at Western standards of income/consumption. Population reduction and/or (inshallah!) space colonization are necessary for humanity to live sustainably. This does not mean that we should segregate the species by belief into "Those who believe in sustainability", who then go extinct from non-breeding, and "Those who believe in having as many babies as possible", who then suffer an overpopulation crisis right quick.

Sustainability yes, voluntary extinction no.