No the mistake was using "Caps" as cash, a century past the destruction of the Hub.
Since the Hub was in California, the
Fallout 3 takes place thousands of miles away in Washington, DC where most people probably haven't even
heard of California, that is irrelavent.
With or without the Hub bottlecaps are portable, almost impossible to counterfiet without pre-war technology, vaguely coin-shaped, and scarce. Thus they make fine money.
Also, in
Fallout 2,
California had indeed moved on to gold coins. But then, it's California. They're practically up to their knees in river-gold over there compared to DC.
Besides, using bottlecaps just feels more post-apocalyptic.