As stated in another post ( http://boards.straightdope.com/sdmb/showthread.php?threadid=99528 ), the terrorists probably cannot destroy the USofA. But basic logic states that no nation can survive forever. What do you think will bring about the downfall of the United States?
Mongol hordes?
Lawyers.
Sloth.
I think we’ll get fat and lazy and get someone else to do our work and we’ll fiddle while Washington burns.
I think we’ll be too occupied with TV and pleasures of the flesh and become lazy and overly enthralled with the status quo of our lifestyle.
And somewhere else someone’s always hungry and not so satisfied with their way of life.
Canada.
Specifically, the anti-competitive activities of intellectual-property, software-patent, and business-method-patent lawyers, IMHO.
Examples from recent years: Microsoft versus open-source. Microsoft versus DoJ et al. The changing of patent laws to extend licensing terms for incumbent IP owners. Jack Valenti and the activities of the MPAA. The attempts by the “4C entity” consortium to embed so-called “copy protection” in hard drives. The short-lived DiVX DVD variant. SDMI. “Food disparagement” laws. “Zone locks” on DVDs. I’m sure there are others.
I fear that the USA is shifting from innovation (which creates new business, wealth, and culture), to attempting to hold onto existing business patterns and wealth (through legal, and ultimately physical, force).
New work will go overseas. Artists, programmers, writers, moviemakers, experimentalists, and other creators will move to locations where they can create new works, and speak in new voices, without being encumbered by legal restrictions and licensing requirements about their subject matter.
The USA will decline in relative living standards as other nations move ahead.
I sincerely hope I’m wrong in this.
Greed.
The USofA will be swallowed up by the Earthian government. Which will of course, also be corrupt. oh bother. 
I would like to purchase a plot on Mars in advance so my grandkids can get the heck off this lump’o’dirt!
Plagues of locusts.
or maybe just stupidity.
Microsoft.
“Looxury and Blasphemy!”
Privates on Parade
I think eventually the US will no longer be a “superpower”, because several other entities on the planet will be just as large, rich, advanced and well-armed. Candidates include a truly united Europe, China, India, a revitalized Russia, and part or all of Latin America. And especially if the US fails to remain progressive and becomes as relatively backward as Russia or Austria-Hungary were in the 19th century.
I most fear the Imperial Chinese scenario: topheavy, moribund bureaucracy; private wealth leveraged out of sight into generations of debt; a hopeless underclass hooked on drugs; and foreign investment-holders making ever more punishing demands.