I once heard a radio host say that disease was "one part genetics, one part lifestyle, and one part mystery" and I think this breakdown is also applicable to analyzing the "disease" or decline of a nation.
The "genetic" part of a nation is the structure of its government, its laws, its military, and the power checks (or lack thereof) which are present in its political system, as well as its geography and natural resources.
The "lifestyle" part of a nation is the morality of its people. The "mystery" part is the factors which no one can control or necessarily predict - natural disasters, unexpected attacks, or just being in the wrong place at the wrong time in history, or, conversely, being the right place at the right time or experiencing a positive reversal of fate.
Although a nation cannot flourish without a moral populace, a moral populace can be limited by the genetic boundaries of its government. The Puritans of New England were a moral people, but their early experiment with a collective economy quickly led to stagnation and starvation. We are fortunate the genetic structure of our governmental system has held up well over the centuries, with, of course, the unfortunate (but hopefully not terminal) issue of the power grab of the Judicial Branch.
As far as geography is concerned, in an era of modern weaponry, our location does not afford all of the advantages which it did in the past, but, then again, this problem also applies to the rest of the world. However, our country still has greatest military force on earth, even though it has been subject to weakening from political correctness. But geography definitely works against us regarding our long land borders with Mexico and Canada. Long boarders are hard to guard under any circumstances, but Washington doesn't even try, so they remain open gateways which are routinely exploited by violent gangs and drug cartels in the south and Islamic terrorist operatives in both the south and north. On the plus side, America has a tremendous amount of natural resources. On the down side, the government currently restricts us from making full use of them.
America is still more religious and socially conservative than its cousins in other parts of the free world. But we have suffered decline in this field, not just over the past fifty years, but noticeably within the past 10-15 years. Who would have thought circa 1999 that by 2012 nine states would permit gay marriage? Or that a place like Colorado would vote to legalize the recreational use of marijuana?
Decades of the relentless promotion of the liberal worldview through schools, media, and pop culture has psychologically scarred the American political psyche. The sum total of which was seen in the re-election of President Obama. I think there is much truth in the theory that voters re-elected Obama, not because they liked what did during his first term, but because they felt certain that he would take care of them and preserve the big government safety net, which provides sense of comfort (albeit a false sense) in our uncertain economic times.
Whether voters realized it or not, they essentially chose security over freedom. It's a choice that people have made repeatedly all throughout history, but citizens of the most successful free nation in history ought to know better.
Playing the "what if?" game usually ends up producing more grim scenarios than positive ones, but it is nice to remember that America's history does contain Providential reversals of fate which have saved our nation from disaster, beginning with the unexpected success of the Revolutionary War itself. Still, one cannot depend on surprise happy endings, although sometimes it feels like that's all you can do.
The painful and ongoing post-election Republican soul-searching has left prominent conservatives like Rush Limbaugh and Ann Coulter coming very close to concluding that the country is lost. I imagine many other people have had similar conclusions. However, there is still comfort in the fact that there are things beyond the control and foresight of human beings.
So, the prognosis of America's decline? Not irreversible, but not good as of now. This is due mostly to the "Lifestyle" factor. After decades of ingesting the junk food of liberal ideology, reducing intake of religion, and being discouraged from exercising critical thinking, the damage has taken a serious toll.