The problem is that Europe has no tradition like we do of taking in immigrants and turning them into “Frenchmen” - “Germans” - “Britons” etc. When I was stationed in Germany in the 1970s I knew more than a few Americans who had married German women and stayed in Germany. Even after decades they were still considered foreigners and second class even if they had obtained German citizenship, which was very difficult to do.
Two points: first we're not doing so well at 'the melting pot' anymore either. Most of the new generation of Mexican and Central Americans aren't interested in becoming Americans. They hold onto their language, citizenship, culture, and habits. They are here purely for economic reasons. Blacks appear to actually be regressing into a more seperate culture than we had 30 or 40 years ago. Think about how few whites have any familiarity with the black television networks, like USA and BET. Rap, ebonics, gangs, baby-mamas ... not exactly main stream USA.
Second point: Germany, with the poor treatment of immigrants did manage to survive as a nation for 1000 years. (And this despite near-constant warfare). What needs to be looked at now is what has changed post 1950, to make the population decline like this?
It is a proven fact that nations based on kinship, culture, religion and land can survive tremendous hardship. Poland was taken off the map twice in the last 300 years, but the Poles refused to be assimilated or destroyed. The held onto their language and culture, and eventually recovered their land. The Jews held onto their language and culture and survived almost 2000 years without a nation-state of their own.
It is an unproven assertion that a completely homogenous nation made up of dozens of ethnicities, with different languages and cultures, often times in opposition to each other can survive for centuries. A century, maybe. Rome did not long survive the integration of the Germanic tribes, nor the fall of the ancient Gods of Rome.
Nations have staying power, empires have less. In so far as America has embarked on the great experiment of being a "propositional nation" and an empire, and is no longer a nation based on blood and soil it's prospects for longevity are greatly decreased. I would be shocked if the USA still exists in anything like it's current formulation in 2110.