No one can predict what exactly will happen in the future. In the event of a nuclear war, they can turn off electricity and the Internet, no currency will have value.
I wouldn't say no currency would have value in this situation, but at one point their will be something that rises above all the be worth something, the gold companies will say its gold, silver companies will say its silver, and so on and so forth.
But yes, at some point all fiat-currencies are going to have to crash and we just have to wait for this to occur. I think that by the time this happens we're not going to even be able to use Bitcoin as we've entered a time where the internet and infrastructure has been destroyed (this is if fiat dies too)