Except for the fact that all things that begin must also end I must wonder at this thread.

Why bother?

   Are you asking whether the government will lose its power? It probably will and then a new government will take it's place. Happens fairly often all tho' the potential to change the government is evry two years.
   Are you asking about the democratic notion losing power? That I doubt will ever happen. Iceland, Greece, some American Indian tribes,
and some other places in Europe at different times in history have held to the notion of a democratic society. One person one vote.
The Democratic notion has disappeared in one place only to re-emerge elsewhere and rebuild on strength of the old ideal with new ideas tuned to the place and time. Read the Irish Declaration of Independence and you read the American Declaration as well as some parts of the Articles that were agreed upon to form the Iroquois Confederacy. The system continues to refine and reinvent it self.
   If you are wondering if and when the American People will lose their power I will tell you I doubt we ever will. We have continually reinvented our selves and our country for nearly three centuries. Sometimes we take awhile to be aware of things but we all eventually can tell Bullshit when we hear it and horseshit when we smell it. And after awhile we get up an do what is needed.
I have said before that we often take several steps back ward but we alway take one step more than we lost forward. We may move slowy but we advance.
The power of the United States has very little to do with the politics at play but it has everything to do with the peoples rights to change their politics and their politicians.
Is your real question when will civilization fall. It never has.
The nature of civilization is also movement and change. The idea of a civil society is another device that can't be usurped or contained.
And civilization is also defined by place and time. Egypt, Mesopotamia, Greece, Timbuktu, Cathay, Persia, Rome... all had flourishing civilizations that flowered died and moved on to another place and time.
   Maybe I didn't understand the question. But I do understand Americans even if I don't understand politics. America will never lose its power because it's people aren't capable of stagnation.
Other countries may come to dominance on the world stage. But America will always be unique and powerful because of the people who live here and not because of the politicians who try and run the show.