What's scary is that it is not a concern to both the left and conservatives in those countries. I noticed the followed talking points when the topic of demographic troubles comes up here in New Zealand. I guess most people in Japan and Europe think along similar lines:
On the conservative side, it is commonly believed that 1) birthrates are encyclical that people see fewer children means more resources for children, so they automatically have more children - a self-correcting mechanism; 2) even if the birthrate declines irreversibly, the increased productivity brought forth by modern economy will more than compensate the economic deficits given that yes, there are fewer people on the net, but we can have more of them proportionately working on higher end occupations. Menial labour demands can be reduced by increased industrial and agricultural automations, and even the unsustainable welfare demands can altogether be solved as a falling population forces the society to move into a more market-oriented direction; and 3) overcrowding will become a thing of the past and resources become mroe sustainable.
On the left, it is believed population falls is a good thing because it means less strain on natural resources and less impact of humans on ecosystems.
Basically, people in these places sneer at demographic disaster talks as "oldtime social democrats talking points". They wouldn't know or care a disaster is coming.