It does, and that's an assumption that probably doesn't hold.
Most things in the economy work that way though - the total supply of hard currency in the economy is only $3.6T, so when you speak of the $18T US GDP or the $20T US national debt or the $524T value of the derivatives market, it necessarily accounts for the same dollar changing hands multiple times. That's what makes it a currency, that it can circulate and change hands while holding its value.
Saying the 'value' of the derivatives market is $524T is disingenuous. The notional value - sure, but that's like saying the value of the lottery 'market' is the jackpot times all the outstanding tickets - http://www.newsweek.com/600-trillion-derivatives-market-9227...
That's my point though - that there are a lot of things in the economy where the total dollar value may be high but it's largely the same participants trading amongst themselves, and the contracts net out to some much smaller value. Bitcoin volume and Bitcoin market cap are not unique in this regard.
No exchanges are collapsing so its about as real as you can get. You might ask 'What if everyone withdraws to fiat at once?' I think if you posit that of any stock market the answer would be the same. Cryptos are a pretty pure free market, if it didnt have real utility or value for people the markets would collapse.
Most things in the economy work that way though - the total supply of hard currency in the economy is only $3.6T, so when you speak of the $18T US GDP or the $20T US national debt or the $524T value of the derivatives market, it necessarily accounts for the same dollar changing hands multiple times. That's what makes it a currency, that it can circulate and change hands while holding its value.