yes,bitcoin has scope of replacing existing money in future,but its not going to happen anytime soon like you mentioned 15 years, US dollar been there for quite a long time and is a powerful currency.The U.S dollar, as the worlds leading reserve currency is money of choice. Approximately 65% of all U.S. dollars are currently being used outside the United States, 80% of trade finance was conducted in dollars and close to 85% of forex trade volume involved the dollar. With so much happening with dollar around the world as seen from these facts,its really hard for btc to push aside dollar and become official currency.