Retail payments are now a little over 50% electronic: http://www.paymentscouncil.org.uk/media_centre/press_release... , so that's bank money rather than paper/metal currency.

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.