Because if FED and US can't do a good job to keep their currency and country running properly, doing austerity when it is needed, not bailing out banks etc. Nobody will trust that currency anymore.

No country in the world can run a $1 trillion deficit or 5%(?) of their GDP every year and not have consequences. This might be "okay" now, but this will create an effect that when people finally start to move away from the dollar, things will run completely out of control in America.