Why is it "alarming?" Japan is the 10th largest country in the world with 126 million.
Japan's fertility rate is 1.41 compared to Germany's 1.45 and China's 1.60. All are below the replacement rate of 2.1. South Korea is even lower with 1.26 and Taiwan with 1.13 and Singapore with .83. The US has a 1.87 rate.
The nine highest fertility rates are countries in Africa with Niger having the highest with 6.49.
Check out the correlation with gdp trends. Granted, gdp isn’t the best metric for economic success, but it’s an indicator.
Japan’s economy remains on the edge of a disaster. As Mauldin puts it, a bug in search of a windshield.
If the population was growing they could work their way out of the problem.