Exactly, when WW 3 happens Bitcoin and Gold will be useless, our currency will be essential stuff to keep us alive like food and water, and maybe guns too 
You know what else is scarce ?
Ammo
Maybe we will even use Ammo as currency just like Metro series
Wars were waged and before and people who had money and gold, had a better chances, this is proven many times through history. Only if you expecting total annihilation than water and food will be main problem and everything else will be irrelevant. Gold will stay always main token for trade!You know what else is scarce ?
Ammo
Maybe we will even use Ammo as currency just like Metro series