"You cannot have diversity of choices if you do not have diversity of population."
Really. So a country that is more or less racially homogeneous cannot have a diversity of choices? I think that is pretty clearly untrue. And what the heck is "diversity of choice" anyway? Either you have choices or you don't. If the number of choices is your metric then there is nothing about racial (or other) homogeneity that limits that.
I think you are putting forward an emotional position and trying to cloak it in the trappings of science/logic in order to give it a credibility that it does not merit.
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"It is only when new races, cultures immigrated here and seeded our society with new ways of thinking that diverse ideas flourished. "

I think you meed to study some history before making statements like that. You might start with the Enlightenment to see how "new ways of thinking" worked out in what was a fairly homogeneous western culture.