I'm going to focus on the population aspect only, so apologies if this is a little off topic from the financial aspect of it:
Just so we have accurate population numbers to consider against this article, here's the Population Pyramid for the United States:
http://www.wolframalpha.com/input/?i=United+States+Population+by+ageLooking at the population data, I don't actually see an issue with our population decreasing like in
Japan.
The two population pyramids are incredibly different. From this data, I believe his fundamental grounds for his debate is false. I don't see a deflation issue on hand. If one does occur, I'd expect it to be minor.
I have no data to back this, so it's just a theory:
To AndrewJackson's example of efficiency and productivity, lets apply Moore's Law to the productivity of human civilization - That every x months a single human becomes x-times more efficient/productive. We are seeing an increase in worker productivity (largely due to technological advancements under the same law) and thus require fewer workers to perform tasks. As this productivity and efficiency increases I think it would make sense that a population would decrease and/or level off as the demand for able-bodies does as well. eg. "I don't need 10 children to run my farm anymore, I have two machines that do the work of 20 people with a single worker. Thus, I don't have 10 children, only 2."
Ultimately, population is also linked to
infant mortality rates. The lower the infant mortality rate, the less children people have, and the population can start leveling off (once we also get away from the Duggars). Combining the two thoughts: "I don't need 10 children to run my farm anymore, only 2, and the two children I do have will probably live. So instead of having 4-5 children (where 2-3 may die in the first year of life), I'll only plan to have 2." This stabilizes the population. One could say that no children are necessary to 'help around the farm' anymore and they may only have one child just to pass on their genes/bloodline/surname - this would be a net decrease.
I have no doubt we will get to the point in the future where people decide to have children because "I need to do my part to sustain my countries population" as one reason.
I don't think this will mean a drastic decrease in the world population (though that would be okay because we are taxing our biosphere significantly - and a natural population decrease due to birthrates is probably the best way (rather than war, disease, famine, murder and forced methods like eugenics)), but it should help stabilize the exponential growth we've been seeing.