The fact that this article offends left-wingers so much by simply mentioning demographic change (i.e. population replacement of Europeans) shows exactly why it was so badly needed. If China can be predominantly Chinese and Nigeria can be predominantly Nigerian, why can't Europe and the U.S. - which was built up by Europeans - be predominantly European? Doesn't mean no newcomers are welcome, that would be an absurd position. But a conservative immigration policy does mean prudence both in terms of numbers and in terms of the choice of which newcomers to allow in, as newcomers with more similar cultural backgrounds simply blend in more easily. If we don't do this, what our ancestors have built up is at risk of being washed away, and this is what we're seeing across the West these days.
Because we are a nation-state, and not a white ethno-state. We don't define Americans by the color of their skin but by the American Creed:
I believe in the United States of America, as a government of the people, by the people, for the people; whose just powers are derived from the consent of the governed; a democracy in a republic; a sovereign Nation of many sovereign States; a perfect union, one and inseparable; established upon those principles of freedom, equality, justice, and humanity for which American patriots sacrificed their lives and fortunes. I therefore believe it is my duty to my country to love it, to support its Constitution, to obey its laws, to respect its flag, and to defend it against all enemies.
Nowhere does it state or imply that you have to have a European background to be considered fully an American citizen.