A country is not a country without borders. When I emigrate I will choose a country based on its singular culture, and crucially, how well it maintains that culture. Why would I choose a country that's right for me now but risks changing in the future when I have kids there?
Multi-ethnicity in London is great. Multiculturalism in London is a sickness. No one chooses to move here for "London's culture," because it doesn't have an identifiable culture. So people come here expecting to maintain their own culture in a bubble instead, resulting in ghettos, self-segregation and rising tension.