Diversity is not something that is good. It does not make a country stronger. In fact its the opposite. Look at Japan, China Russia and Poland. They work hard to keep their country homogenized. And its working for them.
You obviously do not understand the subject matter. Most European countries as we know them today are formed by multiple tribes coming together( war and carnage). Scots, Welsh, English, Cornish,etc The same goes for France and Germany.
You mentioned Japan, China and Russia and Poland.
Do you know why there is a China town in most major western cities? If their system was that great, why didn't everyone stay in China? A lot of wealthy Chinese are investing their money in the US and Canada, why?
Poland, do you know how many polish people work in France, UK and they Netherlands? If Poland was that wonderful, they should all have stayed home.
Japan : Japanese Brazilians are the largest ethnic Japanese community outside Japan (numbering about 1.5 million, compared to about 1.2 million in the United States). They should all have stayed in Japan if the country is that great.
Russia : You know that there are over 40 ethnic groups in Russia and the largest are the Russians. That is why there are always conflicts with other ethnic groups like the Chechens. Homogenized and it's working for them?
if you actually believe that "Diversity is not something that is good" then, all White People should leave America and go back to where ever their ancestors came from. You will only have Native Americans and Mexicans left.
Diversity has its issues but looking at the situation from a narrow point of view doesn't help your narrative.