Japan has close to zero immigration where Europe has a large migration from countries with a much higher birth rate. For Japan this implies that their population will shrink while staying ethnically homogeneous while the European population is likely to increase and diversify. Europe will see more ethno-religiously motivated conflict, Japan will suffer from having a decreasing work force which needs to take care of an increasing number of people in need of care. If Japan continues on this path they'll eventually dwindle as a country and stand the risk of being conquered by one of its more populous neighbours. Europe should have a long hard look what happened in Lebanon when that country was torn apart by a civil war [1] between different ethnic and religious groups.

[1] https://en.wikipedia.org/wiki/Lebanese_Civil_War

Which neighboring country could mount a huge amphibious assault against a developed island nation with still 100+ million people?

The only threat is China and China's aging too...

China, Russia and possibly Korea (which army is far larger than Japan's) come to mind. While Japan still does have 100+ million people the population is going to shrink and those who live there are getting older and older. The low birth rate will make it hard for Japan to maintain a standing army, the ageing population also makes it harder to maintain a reserve force.

Russia has 20 million people more than Japan and its population is also aging rapidly.

Korea? Which one? North Korea? Please :-)

South Korea or a reunited Korea - keep in mind that I'm talking about possible future scenarios, decades from now. There has been and still is a lot of animosity between Korea and Japan [1], this goes back centuries.

[1] https://en.wikipedia.org/wiki/Anti-Japanese_sentiment_in_Kor...