I wouldn't be surprised if you told me that in 100 years we would be merged with Canada and Mexico to compete in the global market. And the Constitution is completely skewered. But that will probably happen long before 100 years.
I have been thinking the same thing. If we are going to stay competitive with the Asian mega-countries (China, India, Russia) over the next century we are going to need a population at least half of theirs which is double what we have now.
I just wish we would be more selective about who we let in here. I have a feeling there will be a lot of immigrants from western Europe as that area becomes dominated by Muslims.