Well, the Chinese, when the current lot ages, will be having the same problem too, but in spades and shovelfuls. Perhaps the Japanese need a spot of immigration to refresh their society. Or perhaps war will make men men and women women again, who seek partnerships with each other for sex, protection and provision and invest in their future by having biddable high-achieving children.
Perhaps a bit of adversity is what is required and more than a bit of adversity is on its way, not just for the Japanese, but for all of us.
What does not kill us makes us stronger.