I immigrated to Korea. It's not quite as hard as Japan (which I've been to many times, since it's like 3 hours away) but I have some thoughts.
You can almost always get what you want. You can even become a Japanese citizen. You just have to work really, really hard for it. But to a certain degree, that protects the culture that you like.
As a Canadian (who loves how easy it is for people to immigrate to Canada), I'm well aware that we don't have the culture of the aboriginal people, or the colonists who came after. That is in large part because of our immigration policies.
Lastly, Japanese and Koreans are kind of racist. I'm actually surprised how well some visible minorities get along in Japan, but I wouldn't be surprised if their day was filled with microaggressions.