A nation fractured by 'multiculturalism' does not work together, and there is no common national interest. Instead you have all the different divisions working for themselves first. You reduce the nation back to tribalism, in effect, and its tribalism thats kept the African nations backward and poor, that's plainly clear to anyone. Furthermore, multiculturalism results in ethnic ghettos, class divisions, isolationism of groups, and ultimately race hatred and conflict, non of which are desirable in any society.
Only when all members of a nation work together with common interests and values do awe all benefit. World War Two showed us that. The rise of the British Empire showed us that. The fact the industrial Revolution started here was a result of it. Commonality is a vital bedrock of a society, and so the integration of foreigners MUST be compulsory.