First & foremost, foreign concerns want to maintain the value of their holdings. Their holdings have lots of dollars, so they'll prop it up. Look at how vigorously the Saudis are propping up the USD. It is not because they "like" us. But because they have too many dollars.
The US consumer accounts for 20% of all retail purchases worldwide. If that stopped, it would be felt from Bejing to Berlin. We buy what they make, if we stop buying, they can't make it, their folks stop working, and get restless. Foreign manufacturers will help prop up the USD.
Race to the bottom...Many foreign countries are combatting the low USD by firing up their own printing press ( currency). They are diluting their currency to keep it from raising in value too fast. Rubles are increasing at over 50% per year, 13% for Euros etc... etc... In reality all currencies are dropping in value (fiat currencies) but relative to the USD, foreigners are trying to keep the balance in place.
There are other reasons, but one over ruling aspect to all of this, a lot of the controls for the USD sits in foreign hands. If someone has bad intentions, they can harm the US tremendously (ie: China dropping ALL of it's USD in one day).
I don't see a precipitous drop for the USD like that simply due to foreign actions, not due to US actions or inactions. If foreign countries stopped propping it up, then I could see it suffering badly, but still not that quickly. Look at how long it took the Zimbabwe dollar to melt down, it drifted down for years before it dropped precariously (as all fiat currencies do).
