The reason no digital currency will ever be used to such an extent is that they are truly worthless. Just because something is rare doesn't mean it's valuable. Also, I've yet to see a digital currency that can scale with a growing economy in order to avoid deflation. That instantly makes hugely widespread adoption impossible, as if a currency starts growing in value, then people will just hoard it, its velocity will fall off a cliff, and it'll just lose the value it gained and then some.
Government-issued currencies have real inherent value because citizens of their respective countries need those currencies to pay taxes, or they'll go to jail. Considering most governments' budgets are at least 15-20% of GDP, with some over 50% (such as Denmark), that is a huge amount of demand for those currencies. In the US for example, every year the IRS takes in ~2 TRILLION dollars in taxes.