The thing not many people seem to talk about is that the 20th century had unprecedented growth in terms of population around the world (which has slowed significantly in the last few decades, although the effects usually lag by quite a bit - when the children enter the workforce and such). We're making up for some of it with technological progress, but ultimately what impact that makes is up to everyone to think about individually (look at Japan as a potential leading indicator of what demographic change can do).