You'd have to refine your question. Bitcoin is already a national currency.
Pontinha recognized it as a national currency back in 2015 (they're very small, located off the shore of Portugal). I don't think that some big country would declare Bitcoin as their national currency and especially not in 2016. The price is not stable enough to act as such.
I wonder how it went in Pontinha and how are the banks located in that place.
Why are you posting when you didn't even read the article that was linked?
Pontinha: There are four citizens: me, my wife, my son and my daughter
There are no banks.
I doubt, even by that year (2026) to claim a label "National Currency" since the government itself, won't let this to happen as much as we like to. However I do believe that it will be replacing US Dollar in terms of most exchange currency in most countries and that would be good enough.
If it gains more stability then why not. It would certainly perform much better than certain currencies. As an example go look up the exchange rate of the Venezuelan bolívar.