It would be pretty difficult to render one of the most combat-experienced militaries in the world ineffective so I wouldn't lose a whole lot of sleep over that.
Americans are resourceful, at least they used to be and if things start to cost too much to import due to a weak USD people will start to make things at home - bonus: US exports will increase since they are cheaper.
Power shifts all the time, but if you think the US will not be a world power in some way, think again. Everyone wants 350 million customers for their crap - the US has that. Look at all the folks trying to get their goods into the hands of Chinese consumers.
True, your paper dollars under your bed and in your bank could become pretty worthless. Stockpile gold and silver and learn how to make whiskey and beer. You'll be fine. Don't buy huge bars trying to trade a good delivery on the street at the end of days isn't going to go well.
It's never a bad idea to have a gun or two for hunting. Learn to reload - ammo will be the real problem you need to worry about. Stockpile that too - it might be worth more than your gold.
Sure, be a steward of the environment that's just good sense and being neighborly so I support that.
If you're really freaked about the end of the good ole US of A, move to Europe.
My motto is "I'm always prepared and if I'm not prepared I'm always adaptable." See if that works for you.