I just read the London article and I'm not sure that everyone would come to the same conclusion. If you've ever traveled to Japan, I'm sure it's no surprise that almost everyone there is Japanese. If, over the course of 25 years, it went from 98% Japanese to say 60%, I hope that we can both agree that it would be noticeable, and that the added diversity wouldn't necessarily mean that Japan is a better and stronger country, especially if native Japanese people were getting arrested for opposing the changes. I think that's the core argument being made, even if it makes for a nuanced and uncomfortable topic to discuss. Not all immigration is created equal either. Large scale immigration (legal or not) can be a financial burden for countries as opposed to selecting letting through competitive talent.