Comment: I will tell you why.

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In reply to comment: Why just Gold? (see in situ)

I will tell you why.

If any medium (less valuable than gold) is recognized as a payment to government, say fees & taxes, then such a money will drive any other types of money from the market. The same will happen if conversion between different moneys is allowed on a fixed basis (or market floating estimations were unwarranted) - the "bad" (cheaper) money will drive out the rest.

The only possible scenario is parallel non-convertible currencies. It makes since to kill paper, but does not make sense to compete between themselves unless a necessity requires as such. It becomes redundant in some way and not universal in another.

PS "drive out" means people will save "good" money for a rainy day while using "bad" money instead.