What will take us down?
Welp, I think it will be something that "everyone" was sure would never happen. Could be small - like some type of viral mutation - or could be something big, like a Coronal Mass Ejection. Whatever natural event that it ends up being, all the "experts" will be just as clueless as the rest of us... either intentionally (to "prevent panic") or unintentionally.
If it's man-made, I'm guessing Economic Ragnarök. Central banks plus Keynesian Economics plus "temporarily" separating the dollar from gold has been a train wreck. We're staring down the bullet of 18 trillion in debt, which is just what they'll admit to. Add in unfunded liabilities and it's more like 100 trillion. The system will eventually collapse under it's own weight, and that's discounting anything our enemies will do. All China has to do is dump 10% of our toxic debt they hold, and that will be all she wrote. Why? I think the Chinese would rather rule in Hell than serve in Heaven... but, all that is for another time....
Or an EMP.
From our enemies point of view, the problem with America is that it's full of Americans. One of our enemies (of which there is no shortage) slips a couple nukes to some 3rd or 4th party. Biff a couple above the ionosphere in the right places and we'd be blown back to the Dark Ages (any of you all read "One Second After"?). Then they can kick back and eat cornflakes... most of us would be dead in a year.
Or, it could be a combination of events... economic collapse sparks conflict and someone pushes a button somewhere... same result.
You all know the old saying: History is littered with wars that everyone was sure would never happen... same thing with collapsed economies, imploded currencies, hyperinflation, etc... everyone was "sure" it would never happen. Only thing I'm sure of is that "experts" are often dead wrong and not to be trusted.
Oh, and one more thing. Remember back when Greece's economy was imploding? One anonymous Greek politician stepped up and said "The job of Government is not to tell the truth. The job of Government is to instill confidence." To this, I would add "by any means necessary"... which includes lying through their teeth to everyone.