>you use it or you lose it
Please explain in direct language the point your deployment of this aphorism was intended to convey as I find it wholly inapplicable.
Exactly for that reason. Note how the Japanese have had their various wars of conquest in the past. At that time they had no problem with their 'stance on immigration'.
Just look at modern-day China: the Han Chinese ethnic group dominates, and oppresses all the minority groups. China is not a country with a lot of immigration either (nor do they have a shortage of people). So far, it seems to be working out for them, though admittedly it sucks for the minorities.
Japan may decide that being self sufficient and self reliant offers them the best return on their lifestyle.