The crime free nature of Japan will be a thing of a past. The quality of life for natives will decrease vastly. Wages will be pushed further down. Having worked together on many projects with Japanese I know immigrants have nothing of value to offer to the Japanese people.

Personally I'd rather live in a homogeneous society having seen how multiculturalism has decreased the quality of life for everybody in my country. I can see why the Japanese would want to avoid that. I hope they revolt.