After everything I said in the previous post, I think we have to look at another fundamental question: is a declining population necessarily a bad thing? In the case of developped countries like Japan, maybe not.
Afterall, Japan is only the size of Sweden or California in terms of landmass, but it has half the population of the United States. For the most part, it is an advanced industrial country which means that it's population uses up a lot of natural resources. Our economic models for development are already not environmentally sound. Our planet can not bear the burden of more and more of the earth's population living Westernised, industrially developped lifestyles of consumption. This is an environmental fact. The earth has limited resources, so it is clearly environmentally irresponsible to tell people to have more babies. Especially not people in advanced industrialized nations where the per capita consumption of natural resources is high.
That being said, there is no ethical basis in denying the "right" of human economic development to people in countries on the path of economic development. We cannot simply say: there are not enough resources, so you can't develop economically. Wasn't the promise of industrial capitalism to deliver us from scarcity with the promise of technology?
But industrial capitalism HAS eliminated scarcity in developped countries. It has in fact done too well. Through marketing and the vicious cycle of "trends" and "fashion," scarcity and want are "manufactured" as well. We must have more, consume more. Support more industry. This is industry for for it's own sake and for the sake of capitalism, not for the people's sake.
Yet there are parts of the developping world where industrial development has not yet made the significant inroads to improve the standard of living. For the less fortunate, the promise of technology has been a betrayal. Technology, while it has improved the quality of life of millions, is still primarily in the service of capitalists to allow them to make more money.
Thus the only solution, given limited resources is to have fewer people. Ethically, we are compelled to spread the wealth, but the only way for that redistribution to be environmentally viable is to have few people around to spread the wealth too.
On the political and cultural front however, an aging population also means a natural tendancy towards conservatism. When one-third of the population is eldery, they will have huge sway over government budgets and agendas. This will in turn take money away from education and government programs to help the young. Young taxpayers will no longer be working for their own future but instead paying to support their elders' pentions. This could pose a serious handicap on further innovation on all levels, from the arts, to public administration, to technology.
So I guess I only have more questions and even fewer answers for this difficult topic for Japan and many other developped countries. So a net population decline is not necessarily bad for the planet, but there will still be serious political and cultural ramifications in societies full of old people. At the same time, measures should be taken to allow for women and older people to work and for the underemployed to find more meaningful and profitable employment. Measues will also need to be taken to make sure that technological advances allow for more ecological development and be redistributed to benefit more people rather than just a select few. A difficult task indeed.
So even if the population of Japan ages and declines and the gross economic output declines, structural and technological changes can be made to increase production efficiency, eco-friendliness and to help maintain the high standard of living.