Also, the vast majority of immigrants to Japan will always be from other Asian countries. It seems like a stretch to assume none of their neighbors (from whom Japan adapted much of their own culture) share anything in common with them.

But of course, this isn't about Japan. It never is. This argument is always an orientalist proxy for Western xenophobia and racial prejudice. A polite way to say "I don't like the brown people in my country who speak a different language and worship strange gods, wouldn't it be nice if we could be more like the noble Japanese, who are willing to fall on their katanas and go extinct rather than suffer the impurity of foreign blood?"