Aside from the rather irritating concept that the world was put here (by some deity?) to produce the absolute maximum quantity of happy humanity, it seems to me

Aside from the rather irritating concept that the world was put here (by some deity?) to produce the absolute maximum quantity of happy humanity, it seems to me that the main fallacy here is that things remain the same with a population reduction. I can guarantee that a 10:1 or 100:1 shrink in humanity will result in a society that is nothing like today's. Any prediction based on the interactions between 8B people won't hold when it's 800M.