Japan's rising sex aversion (2012)

We don't know the cause, but environmental hormonal causes are one hypothesis.

Here's one aspect: https://en.m.wikipedia.org/wiki/Testicular_dysgenesis_syndro...

It's easy to see how lower testosterone would lead to less sex. It is very unlikely to be the only cause, but it likely has some impact.

What China wants is for it's hegemony to be recognized.

Why invade Japan and risk it being highly unpopular at home; being sanctioned by every major developed economy; militarization of it's neighbourhood, etc. etc. (on top of actually being unable to actually conquer Japan)

Within 20 years, it could be well within the position of dictating Japan's foreign, or even some domestic, polices to such a degree to have it essentially as a vassal state. Japan perhaps willingly chooses to make certain concessions in return for favourable treatment for China - say if the two countries considered closer trade relations at one point.

There's also diplomatic & soft power. An aging Japan turning to immigration which de facto will largely result in an influx of Chinese (already the largest foreign minority in Japan). These chinese immigrants over time have more power within Japanese policy.

If China manages to launch it's own cultural industries to the degree of S. Korea's or Japan's, that would make room for more Chinese influence.

But in the end it works both ways. Young Chinese imitate certain aspects of Japanese culture as well. It doesn't really need to end in war or even aggression for both countries to slowly learn to have a meaningful relationship with each other.