Forex trading is huge: http://finance.yahoo.com/news/forex-market-size-traders-adva... : again, held on account rather than actual notes and coins changing hands. That's not even counting derivatives, but derivatives usually net out (for every put option there is a call option) and don't necessarily require settlement.
However, there's enough of a real market and taxation demand denominated in the US dollar that it's not going anywhere any time soon. The ECD made enough of a commitment to Euro liquidity that it's not going away any time soon.
Bitcoin's market cap is still tiny enough that it could be demolished by a single bored FX trader in an afternoon; but there's no real way to profit from that, as doing so would also demolish the fragile exchanges by which you might cash out.
I'm talking about the trillions of derivatives market ..