Okay, but like, Will Japan be more Japanese when the last fully Japanese person dies, or will it be more Japanese if people move and live there and melt into the culture.

Yes, there will necessarily be parts of your culture that get lost (in Japan's case it will be the part where they don't like immigrants), but not as much as if you fold people into the culture and teach them about it.

And yes, change is scary, but sometimes it's good for us. Sometimes change makes us more resilient as a species. Also, Japanese culture isn't going to disappear just because they allow immigrants in. In the same way that other cultures haven't disappeared just because they allow immigrants in.

American culture used to be so exciting because you can experience any kind of culture in the world just by meeting your neighbors. I miss meeting new people and experiencing their local cultures through them.