Digital currencies would more likely be the future but we are lacking assurance that it would be bitcoin that will represent what future would look like, since there are other digital currencies being more widely used by this time. It would mainly depend on how will its futher developments become.
Digital currencies aren't the future, they are the present - online banking and internet payment companies were around for decades. Yes, they use existing fiat currencies, and it's okay, there's no reason to create a brand new currency if you can use the existing and well-adopted one. Of course it doesn't apply to crypto, because you can't really put an existing currency on a decentralized blockchain, all stablecoins will always be centralized.
I'm referring to the cryptocurrencies, not fiat digitally produced currency.