As much as I love Bitcoin I think it's unlikely for
all fiat currencies to implode at the same time, at least anytime soon. For now any collapsing fiat currency will more likely lead to people fleeing to other fiat currencies. This is unlikely to change until Bitcoin starts to play a larger role in international trade. If that happens, the clock will start ticking
fast.
My fear is for the masses...It's a scary thought to think what an implosion of fiat will do to those who have no bitcoin, or who have debt, or worse, both.
Upside: In your scenario having (fiat-denominated) debt is a good thing, actually.