Japan has many problems but in the long run their demographics will work themselves because they are not going to let any foreigners into there country. Japan for the Japanese. They believe in borders.
As the population in Japan drops, the cost of living will drop too. There will be an equilibrium point when people start having larger families again. This stuff about a need for an ever expanding population is nonsense.
Above all, it’s GDP per capita that matters, not total GDP. You increase GDP per capita by increasing the average productivity of each worker, not by adding workers from low productivity societies.