The way I understand how a negative scenario for population dynamics could play out in Japan is as follows:
Japan has a large segment of its population that is approaching retirement. When all of those people retire, they expected to be supported by the working population which is now far too small to support the large number of retired persons. Of course, theoretically those retirees should have some significant savings since we've been informed for years that the Japanese are great savers. So the burden of supporting a large retired population with a small labor force might be delayed for a few years. On the other hand, the use of savings by the retirees forms a wave of dis-investment that could torpedo efforts to offset the shrinkage of the workforce with robotic or mechanized production(of which much has been made in the press).
Political and social division ensue. Leaving aside the options the Japanese government has for handling that situation, pretty soon thereafter members of that retired group start dying. In 2006, according to PRB.org's World Population Datasheet, there were 9 births for every 8 deaths in Japan. When the retirees start dying off, that ratio reverses and could go fairly high in the other direction, say 12 people dying for every 8 born. So massive population shrinkage ensues.
Since the population is shrinking, domestic demand naturally is shrinking as well, and therefore Japan becomes even more dependent on exports for positive GDP growth. In reality, once the ratio of births to deaths goes much below 1.0, positive GDP growth is not possible. The shrinkage in domestic demand will be too much for any increase in exports to overcome. And when the yen's value versus other currencies eventually rises to a realistic value, exports will drop off as well.
Since the population is shrinking, real estate prices decline steadily as well. So there could be a negative home equity effect on consumer spending.
Another major problem will be the large national debt built up which now will need to be covered by the output of a much smaller labor force. A default by Japan on its sovereign debt could occur.
Did I leave anything out?