I don't think they changed the core philosophy of what it means to be Japanese. It seems to us a radical change, because what happened was comparable to the unification and industrialization of Germany, except that the process was much more rapid. By 1894, Japan already showed its power against the Chinese, even though in the 1860's they were relatively backwards, comparable to a medieval feudal European kingdom in their structure. But I think it would be fair to say that the essential principles for Germanic peoples remained the same from what they had been centuries ago, and the same could be said for the Japanese. Nationalism was only truly effective in uniting folk that shared a common cultural, philosophical, linguistic tradition. Attempts to unify people who did not share these qualities, usually ended in failure. For Japan to suddenly turn into modern-day Germany would be a bigger leap of faith. They simply don't share the same sentiments of what WW2 meant for them. Japan still retains the same philosophy towards social structure, while the Germans believed that they needed to rebuild everything from scratch (Stunde Null), including what it means to be German, which means that they've taken active measures to abandon the remnants of the thousands of years of historical, philosophical and cultural development that had brought them to this point. For Japan to adopt immigration as a policy, would require them to critique their social structure and cut themselves off from their history. I cannot see that happening.