ManfromMiddletown:

If demand in advanced countries declines with the population, then who buys the stuff.

If the premise of our economic system is that we must have ever increasing production and consumption we are in trouble anyway. Finite world and all that. Further, I would argue that Global Warming and other environmental calamities are if nothing an indication of over production. The greenest product is the one you don't buy. The proper challenge is to figure out how to have society work less and consume less, not to prop up consumption in face of a falling population.