Why? If American Indians were capable to unite and push out the invaders/settlers when numbers were in their favor it would be OBJECTIVELY a smart and right thing to do.
Same with the later non Protestant immigration, limiting incomers to Protestants (with preference for the British) and relying on already high domestic birthrate would keep USA stronger and longer lasting.
Of course it would be much more boring place and we would not have Godfather movie, but you cannot have everything :(
Probably people like me would not be allowed in, but this should not cloud our judgment.
Do you want America to be a global market place without national identity to end up like Rome which fall apart into separate nations ruled by barbarian tribes?
Eh, whether I want it or not, history will play out as it does. I won't be here to see it.
I have eight children - not many have done more on the demographic front. We like Mexican better than Italian, but that's just a personal thing, como no?