Not just Japan. Stats are pretty miserable across the developed world.

The main reason for demographic decline and low fertility is liberal consumerism. Liberal consumerism is the religion of the developed world, and like all religions, it is a worldview that shapes one's understanding of what life is about. Consumerism's implicit anthropology is hostile to fertility, because fertility is at odds with the consumerist imperative. It also shapes how people view relationships and society. Consumerism is totalizing and produces a culture that smothers everything in the logic of consumerism.

Immigration is just an extractive and parasitic bandage over a gangrenous limb. The solution is to destroy consumerism and replaced with something better and more human. This will happen sooner or later, as consumerist societies will be eradicated through selective pressure (they'll go extinct), but it is better to voluntarily wage a religious, cultural, and political war against consumerism to save these societies.