OK, my math may be off in terms of how much is actually needed to reach that mark, but say the market cap reaches $30 billion, is there actually $30 billion worth of purchasing power? Until people/businesses can start directly taking btc and not needed to transfer to fiat, then I don't see how the market cap can stay at a level like that.
Same goes for all markets. How long would gold's market cap last if everyone attempted to sell at once? Apple would sure as shit not be the world's most valuable company if every shareholder decided to divest.