Currecy that depend on fiat anyway can't replace it, and bitcoin also can't become fiat currency, cause in many countries it's still illegal for using.
Internally, Bitcoin doesn't depend on fiat, it is self-sufficient and thus doesn't need validation from dollars or whatever. On the other hand, since Bretton Woods agreement and till Nixon Shock the US dollar had been officially pegged to gold, but it didn't just perfectly replace gold as a currency, but actually moved gold out of circulation altogether as a monetary unit...
Gold bullion is now used as a substitute instead of gold coin