Once a sufficient critical mass of citizens is achieved (you've see the same reports as I of massive inter-state migration), then the history of manifest destiny will merely be replayed as territory is reconquered.
No one ever learns from history - it simply isn't in our DNA. We must repeat the same steps & processes of our fore-fathers over & over again to achieve the same damn result.
Once we decided that America was a “proposition” nation which could be multicultural and still be “America”, we were doomed. America was an extension of European civilization. Our Founders took ideas of liberty, rights, and the rule of law that had been germinating in England for a long time, and because of our geographic isolation from the crown, put them into effect. The idea that our nation can fill up with people from alien cultures to the point that they are the majority, and America will continue as the great experiment in liberty that it was, is a fatal error.
A small number of people from other cultures adds some spice to society and they can be assimilated. Mass immigration from alien lands not only cannot be assimilated, but will eventually subvert the founding culture.