I think the world would be much more happier if we were all like Japan. Homogenous societies who focus on themselves first, instead of importing foreign people who pose a threat to culture and cohesion.

It's axiomatic that people that are more similar will have greater agreement. Conflict destroys prosperity (abundance). Diversity necessitates people having less in common with each other and thus more conflicts. Thus diversity destroys prosperity and the solution is to return to states with greater (optimal) homogeneity.