So we will need another $2,000,000,000 just to maintain current prices?!
...
That's not how markets work. Market-cap and flow are entirely different things. For example, if the last trade was $630, and there's a 0.10BTC sell order at $632, and it gets filled, someone spent $63.2 to bump the market-cap from $8,190,000,000 to $8,216,000,000. So $63.2 in trading upped the market-cap by $26,000,000.
Obviously markets are usually more balanced than the above (lots of orders getting rapidly filled from both sides of the book), but the point is basically that if you have a thin market, tiny amounts of money can move the market cap significantly, so you can't really make statements like yours above.