So what happens when the exchanges run out of bitcoin, low liquidity, then the Altcoin bubble bursts and all the alt-loving newbs frantically run for the trade back to btc? $30B trying to get back into btc all at once?
Marketcap has nothing to do with the amount of traded coins.
Only a friction of each altcoin (and bitcoin) is traded. Most of the coins are lost or in cold wallet.