The total number of people living in the world does not matter to local areas that see themselves age rapidly and hollow out as young people leave and they become unable to support the generous welfare we give to the old.

I don't know where you got that idea. Some species critically depend on wildly unstable birth rates (grasshoppers and cicadas, but probably also deer and many other prey populations).

Stable populations are completely irrelevant at the microscopic levels; InBev would fold within a week if yeast populations were stable.