If Generation A has a higher number of people than Generation B then when Generation A dies off there will be a lower number of elderly in Generation B. It’s a temporary slump. It might last a decade or more, but it is temporary.

Right now a lot of skilled workers are fleeing to the EU, so Japan could totally capitalize on that. Or it can just educate its population to be skilled labor and give all the low skilled labor (if that even exists) to immigrants. Immigrants work hard for lower wages and are less prone to crime, there is no good faith argument against that.