Your argument implies that young immigrants will somehow find it affordable to have babies in Japan when Japanese citizens do not. This is perhaps true for skil

Your argument implies that young immigrants will somehow find it affordable to have babies in Japan when Japanese citizens do not. This is perhaps true for skilled workers? But unskilled Japanese workers cannot fill that demand, and without access to a wider pool of skilled labor the Japanese industry will continue to stagnate. Another factor that you didn't consider is that prices dropping due to economic decline doesn't make life cheaper for people who live in that same economy. Their income is of course linked to demand on the labor market. And last but not least, population can also drop due to expatriation, which inevitably grows in times of economic decline.