So will every country eventually. Usually countries solve this with immigration but Asian countries (as opposed to Western ones) are quite xenophobic based on personal experience and those of talking with others. As such, they don't let in many people and those that do find a hard time becoming a citizen (Japan's resident foreigner population is 2.29% according to Wikipedia).

There is also not a huge reason to have children, I think many are finding. The societal pressure from decades or centuries past is simply not there much anymore. They already have jobs, are making money, so why spend it on raising a child which can be very expensive? Or they're not making enough money to raise them (see rising rent prices and stagnating wages), so again, why spend all that money they don't have on raising a child?