Fiat money has fallen down many times in failed countries, failed government or just failed economic policies most famously Zimbabwe.    So I wouldnt say never, essentially fiat is a weakness foisted on the public by their governments for convenience of taxation is basically why.

It could happen worldwide that all fiat currency becomes distrusted.  If we can have reserve currency we can have the opposite where people have a great preference for gold or some solid standard and bitcoin could be that.   I just think its unlikely we find bitcoin is the global standard, it would have to be a people power thing .
  The reason why gold was a standard world wide at one point was for the confidence it brought in trade.  Between them governments did not trust each other and demanded gold for international trade, in ancient times Roman soldiers demanded gold in threat of revolt but the people themselves were given fiat.   As recently as the 1970's the French refused to take dollars and demanded payment in gold citing currency fears, even now France has some of the largest reserves for its size afaik (but also much debt now)
Now governments are willing to swap dollars between themselves and with alot of politics each government passes down the inflation resulting to their people.  Sometimes this leads to failure such as in Egypt where the people saw their bread rise rapidly in price, its not impossible the current dollar standard could break worldwide