They might finance retirement of chinese, though. Immigration is nice, but it's far from the only path to sustained economic development.

Or at least, not without very substantial immigration.

Of the former children from Africa, Syria, Afghanistan etc. which came to Germany (Europe) in the last three years a too high percentage does not seem to have the ability (or willingness) to replace a worker in Germany. Immigration alone doesn't help. It may even make matters worse if the 'wrong' people come. (I wrote 'seems' because it is difficult to get objective hard numbers for this emotional subject).

Regarding birthrates/immigration: Help without conditions is wrong, I now think. NGO's should not be allowed to 'help in Africa' (and Afghanistan, ...) without making sure that the birthrates drop significantly. Providing modern medicine without enforcing the 'recipients' to adapt the culture/custom will lead to a desaster in those countries (and if 'everybody' migrates - in Europe also (will happen after my lifetime probably)).

When immigration is discussed, it is important to separate refugees, illegal immigration, and legal immigration.

Being an island nation, Japan is in a good position to control their borders tightly compared with other countries like Germany.

It then becomes a question of having a well-designed immigration process that selects young, healthy, highly-educated folks who, most importantly, must speak the local language fluently. Canada has a pretty good system in place for this.

Why do you think it's okay to make African children pay for elderly Japanese and Europeans? Why do you assume they would even want that? They are not instruments born to serve your desires.