If the USD becomes worthless you won't be able to buy anything with it. Gold, food, guns, gas, nothing...
I'm not sure what you mean by "commodity money", but at some point if the world devalues all their fiat currencies to zero we'll have to go back to swapping gold/silver coins, bartering for whatever we can get from each other and do things the way it was done in the not too distant past.
The ponzi scheme can only go on until it doesn't work anymore and everybody realizes what's going on. Just my take on it.
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