Who is swimming to Japan? Joking of course, but you have to consider that unless there are land adjacencies, it’s unlikely invasion is a concern as population declines.
Less Japanese means more open space and less land contention from remaining Japanese. Less people overall means less resource contention overall.
People will be boating to Japan as countries in East Asia plus Russia are destabilized in the next couple decades due to rising energy and food prices.
Less Japanese means more open space and less land contention from remaining Japanese. Less people overall means less resource contention overall.