Uh...
I think only time can determine. I personally don't think the US itself will just die out all of a sudden. There are a lot of people of different backgrounds that date back to the 18th century, and they lived here for a long time. They would move out or within the country trying to look for more benefits to their life and well being, working much like blood pumping through the body. I don't think this will stop anytime soon, since a lot of people are always on the move.
If it was anything that would at least bring America to a grinding halt or just back up the normal way of life, it would be some stupid fault by the government for the economy, not really by the people. There's a huge amount of debt in the country as it is, $170 trillion, and more is just being blown right out the window.
Let's envision some hypothetical future event. The government could one day set the taxes to a rate that could be simply atrocious if they fail to create enough currency without generating inflation, which is impossible (In this scenario, let's just say 100% taxes on the upper, middle, and lower classes. I know this sounds ridiculous, but I don't see why this couldn't happen. Maybe a greedy overlord or diehard tryhard for the economy would take over as president one day). They try to lower the debt a bit. Some time after these standards are set, people could start being confused and angry as to why this is happening. In a moment like this, it's pay and have very limited commodities, or give up the building you run/live in and lose about everything.
The businesses wouldn't have anybody buying anything anymore, so the small businesses would start laying off workers for budget cuts, but no one would be buying anything anymore, so they decide to shut down. The bigger businesses controlling those smaller businesses would do anything they can to maintain the process of keeping production up with the small amount of workers they now have, but it's no use since, once again, no one is buying their stuff, and therefore no funding is going through, and it's useless to build things without funds. They shut down. Same goes for farms. They aren't funded for growing plants anymore, so why should they grow stuff for people who aren't going to buy anything?
So after everything has been foreclosed and discontinued, the government could try to reconsider the decision and lower the taxes once more and try to take it easy with all this money, although it's futile since many would lose their jobs. Even trading is useless. They couldn't think of anything but producing more currency with the risk of inflation. They are running out of ideas. Some time later, it is too late. America is suffering another Great Depression.
The people have had enough of being forced out of their homes, waiting in bread lines and sleeping in shacks. With enough time, they start to take up arms, using anything they could find, and march right to DC to riot. Policemen that still have jobs try to control everything, but they fail. The people want their jobs back, money to buy their favorite things and consumer goods. The government finally succumbs to this and collapses because of the choice they made.
What happens from that point on depends on what the people or the remainder of the government do. I won't think about that too much.
And I spoke of politics.....sorry.