A smaller population allows you to devote more resources per capita while reducing the resource consumption of the population as a whole. Apart from the obvious appeal of enjoying at least comparatively luxurious allocation of public resources, this can have real benefits for a modern knowledge economy; having more educational resources to devote per-child can give your country a leg-up on those that have to use their funding on a much larger, ever-growing student body.

Put another way, which problem would you rather have: desperately trying to keep 2000 inner city youth from dropping out of a school that is halfheartedly going through the "everyone should go to college" motions and not really teaching them anything relevant or helpful to their life track so that the majority can graduate into minimum wage service jobs at best or end up on welfare or in a gang at worst, or teaching 20 everything you know about web development or some other marketable trade that can lift them out of the vicious cycle?

I don't really know anything about Japan's education system beyond serafuku, but this is the way I see the world moving in the somewhat near future as manual labor and service jobs continue to fall to automation.

The birthrate problem is advanced nations is actually a real estate problem.

Young couples can't afford a single family dwelling within a reasonable distance of job centers. As a result they rent apartments and buy cats.

A declining population eventually forces the government to create a sustainable housing situation.

Less demand for resources, and consequently less damage to the environment?

> Less demand for resources, and consequently less damage to the environment?

While this is intuitive, it is largely incorrect. As our population has increased, we have become more efficient due to network effects. With a decreasing population, you'll experience a reduction in efficiency.

Also, at what point will you be satisfied? Ideally, with humanity extinct, we'll be using zero resources and won't be damaging the environment at all. That's the definition of misanthropy, isn't it?

> As our population has increased, we have become more efficient

The efficiencies that we've gained hasn't kept up with population growth. If it did, we wouldn't be having major issues with things like climate change, over fishing, or water shortages. Moreover, if you've read one of Bill Gates' more recent blog posts; gaining efficiencies doesn't lessen demand for resources.

http://www.gatesnotes.com/Books/Making-the-Modern-World

> Also, at what point will you be satisfied? Ideally, with humanity extinct,

Please re-read the post that I replied to. I was just merely giving an answer to a question: "Why is a smaller population a good thing?" You're making too many assumptions with too little data.