If Japan doesn't want to go into a death spiral as it's population shrinks they will need to get used to the idea of immigrants being common, and being a Japanese citizen is what makes you Japanese.
I am very mixed race, including 18% native american and 24% west African (slave). That's according to DNA analysis. According to my grandmother's genealogy efforts, the only member to immigrate to the US after 1776 was Irish and he married the "creole" (white+black+native+???) plantation owners daughter. Resistance was futile. He was assimilated.
So most of my bloodline has been here since the start, 18% before that, but because I am not white and my features' origins are hard to define, I get asked where I am from fairly often. I've been even told to go back to my country.