Bitcoin has the potential to be second behind gold, because we see the development and movement of bitcoin so far. but we know everything takes time. I don't think bitcoin will overtake gold yet, Gold is an investment legend. it is too difficult to replace gold in physical form, while bitcoin is a digital currency.
It's not going to be an easy journey my friend. For Bitcoin to cross reach that market cap, if all Bitcoin are already mine I.e the 21 million maximum supply of Bitcoin, If we are to divide $15T by that number, we should be having Bitcoin price at an average of $710k which you and I know that it's not going to be an easy journey and that's like 100 plus year we are talking about.
We don't know what Gold price will be by then, if Gold double it's price by then, we should expect Bitcoin should an additional 2x which is going to bring Bitcoin to another $1.4M before we can achieve such price. I just think there are some things that is not possible and this is one of it, maybe things might change tomorrow.