My jarring awakening to the importance of genetic and cultural Japanese-ness in Japan was when that girl won the Miss Japan pageant. She was seen as not-Japanese as she was white. Which I hadn't thought would be a problem. (Maybe it was foolish of me, but to be fair they had actually already had a black Miss Japan. So why wouldn't they accept a white one?) Anyway, it culminated in that girl giving up the crown but it taught me an important lesson after several conversations with friends.
Japanese-ness is not some backward notion from a time that the world has advanced beyond. They take Japanese-ness extremely seriously. One ignores it to their own detriment. As that particular Miss Japan contestant found out.
And as, I suspect, many foreign residents of Japan will find out in the coming years. It's unfortunate.
But culture above everything I guess?