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