I don’t think the parent you responded to meant by depopulation the TOTAL extinction of humans. It’s likely they meant, say, massive increases in family planning and women’s education leading to people having fewer kids later. Child mortality has been greatly reduced, someone should tell many African men! Religion plays a big role in this as well, around the world.

We have learned how to live on a credit card at the expense of our children. We imagine we have “escaped the Malthusian trap”, but the bill comes due!

More people means more factory farms, overfishing, desertification, deforestation, plastic pollution, garbage and releasing greenhouse gases. The accumulation of garbage and the depletion of resources can throw delicate ecosystems out of whack (bleached corals, for instance, or collapse of bee colonies).

Ar best, we are reducing biodiversity greatly and turning the earth into a monoculture of fatory farms for feeding a growing population of humans.

At worst, that’s one unsustainable step away from collapse.