You'd have to change the entire foundation of what enables the Japanese society to function, in order to make people accept immigration. Otherwise you're just setting those immigrants up for failure, to become ostracized second class citizens, and possible figureheads to assume blame for the society's failures. It's obviously a more complex philosophy with many varying takes on the subject, but in essence, Japan, and most Asian societies around that region, are built upon the ideal that one must humble themselves as an individual, to become a part of the greater collective. And we are obviously not talking about a collective in the loose definition of the term. We are talking about a very strictly defined collective of Japanese people, sharing a likeness in thought, appearance, objectives, that the individual feels obliged to sacrifice himself for. This social pact, and the level of trust involved, is broken the moment that the Japanese is no longer surrounded by people who share his thoughts, or appearance, or goals in life. Not that there aren't disagreements among the Japanese themselves, but there is a general agreement that everyone is playing for the same "team". Something that is impossible to duplicate with immigrants from an entirely different culture, subscribing to an entirely different system of beliefs, who look, act , and talk different from the Japanese. By breaking down this structure, we're essentially going against thousands of years of philosophy, culture, tradition, that has allowed the Asian societies to exist and prosper in their current form, by having hammered down the nails that stuck out. The result of such a social and moral collapse could be far worse than any negative results from an aging population (which can still be addressed within a population, by adopting the required policies, without relying on the import of populations from elsewhere). I don't really see any reason to "poke the bear" in an island that is already scarce of resources as it is, not to mention a prime target for a major natural disaster.