It's not just Japan, the underpopulation crises is hitting many nations. It is even used as an excuse to flood nations with immigrants who have values antithetical to those of the current population.
When the popular book "The Population Bomb" came out, over 55 years ago, the world population was about 3.5 billion and we were way overpopulated. Now the world population is about 8 billion and we are underpopulated. Care to do the math?
I am old enough to remember public service commercials that had messages like: "the easiest way to fight the population crises, is to have smaller families." It was about that time that China started their "one child per family" policy.
It's my understanding that, before 1945, Japan was desperate to acquire new territories because they believed their island nation to be overpopulated. I wonder what Japan's population was then, as compared to today?