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This argument seems to have an unsavory consequence. There’s nothing wrong with excluding and persecuting minority groups because they’re not part of the nation.Minority groups have been part of different nations for millenia. So saying "they're not part of the nation" is not a given. They're just not be part of the majority group of a nation. Some nations might not even have such a majority group, but be composed of two or more different groups of roughly equal size. Nations aren't monolithic things. But that doesn't meant they're "anything goes", either.
Nor does "a nation of people is more than just replecable individuals" has "it's OK to exclude and persecute minority groups" as any kind of necessary consequence, anymore so than "this person doesn't belong to my family" means we can mistreat it.
It does however mean that "just bring foreigners in huge numbers to increase the local population" is not exactly a great solution. It might even amount to ethnic cleansing, and this has happened in the past too.
Historical immigration was often part of such power plays and land grabs (like against Aboriginals or Southern Africans), or "population engineering" (those in power bringing in populations to divide and conquer, and to replace and suppress local populations). In fact one of the common targets that was yielded against was exactly minority groups local to a nation or part of an empire.