China's economic growth simply isn't sustainable, and that's even if it was true growth. There isn't a top economist alive who believes China's self-reported GDP numbers aren't fudged in one way or another (source: http://www.businessinsider.com/economist-reactions-to-chinas...). Either one of those situations (an economic slowdown, China fudging the numbers, or the more likely situation: a combination of both) makes hedging against the Yuan a pretty solid bet.

People and foolish investors think, "Well, China has more than a billion people so it will eventually have a robust and rich economy!" Only, it doesn't work like that (cue the "That's not how this works. That's not how any of this works" lady). Population size is only a tiny facet of a much larger picture. And it's not even a particularly important one, relatively speaking. There are some economists who are now starting to think that having a massive population size may have the opposite effect; it helps you reach a certain point but then your massive size begins to act as an anchor which keeps you from growing further.