The real issue is the cost/barrier to entry. I'm pretty sure Japan would be full in 4-5 years if they would open the gates(i.e a free border with the world). It's not like the immigrants are looking for cultural assimilation. Maybe their children but that's not a sure thing either. You can see that in Paris and London where even the 2nd or 3rd generation is still living within the same "communities" with little cultural assimilation. With enough mass immigration you may see a reverse assimilation.