I don't think bitcoin will be a true currency. People are not going to use it to buy grocery, an apple, or a cup of coffee, because it's already easy to use cash to exchange for everyday goods, how could bitcoin makes it easier. But btc can solve other problems, like transferring wealth from bank to bank, or across countries, across planets, the fancy contracts, replacing paypal and credit card, replacing precious metals, increase anonymity when dealing with porn, pros, drugs, etc.. So I think the market cap of bitcoin can't be as large as cash, but definitely will be more than a grand of dollar,