But if we're going down that road, Second - America has been absorbing and integrating millions of people throughout its history and has only been the wealthier for it.
"The right of asylum (sometimes called right of political asylum, from the Ancient Greek word ἄσυλον[1][2]) is an ancient juridical concept, under which a person persecuted by his own country may be protected by another sovereign authority, such as another country or church official, who in medieval times could offer sanctuary. This right was already recognized by the Egyptians, the Greeks, and the Hebrews, from whom it was adopted into Western tradition."
Whites don't have to be in a majority to cause misery :)
Exhibit A: South Africa.
More specifically - even if whites continue to constitute a smaller and smaller portion of the US population, it is likely that whites will continue to have an outsized influence on the nations politics and economy through entrenched power structures.
The article's headline (and much of the text) is pretty vague. The astronaut apparently doesn't think we will get there "soon" - whatever that means.
If we look at it in perspective of all 5,000 or so years or recorded human history, I'd say we WILL be there pretty darn soon. Just maybe not next year.
Older people are not as productive or innovative as younger people.
Therefore, there will be fewer things produced and less to go around for "the remaining people."