Gold and bitcoin are kinda complementary. Bitcoin strengths are golds weaknesses and golds strengths are bitcoins weakness. For example, gold is stable, whereas bitcoin is volatile, combining both is the best way forward for those who can afford it.
Bitcoin and gold are fundamentally different things. And bitcoin can never replace gold. Because the main idea of bitcoin is decentralization and the lack of physical value. So bitcoin can not be the basis of the world economy.
Currently, it can not become world currency, however, as technology grows to its best, when people work with technology, everything will be paid with bitcoin or any other currency.
Rightly said it may takes some decades to become the world currency beacuse each country is not the same page regarding the cypto currency. But as more and more countries starts to legalize the currency we will have more people using and making payment through this currency. Also it helps in remittance so this is also an advantage to everyone in the world.