Since bitcoins are denominated down to 8 decimal places, there are actually many more than there seem to be. For example, if we did use them exclusively and had a desire to keep our global currency level equivalent to trillions of 'units', we would see the following exchange rate:
21,000,000 whole bitcoins
Each one can be broken down to .00000001
This yields 2,100,000,000,000,000 units or 2 quadrillion.
Accounting for the smallest to equal 1 current penny, we now have 20 trillion dollars worth of them which is roughly half the global GDP.
So, if this took place tomorrow, we could buy anything with bitcoin down to 2 cents worth from a direct exchange. We would just have to label them with prefixes like milli, micro and nano. So today's "2 pennies" would equal 10 nano-bitcoin and a whole bitcoin would be worth $2 million.
(Note: The exact number of them is 2,099,999,997,690,000)
EDIT: I just realized that I assumed you proposed bitcoin as the world's RESERVE currency, not the only currency used in the world. Being a reserve only serves to set the standard of what the other currencies are 'officially' worth. If used this way, we would only need maybe 1/10th to 1/1000th of the amount I noted above. So the ultimate value might stop ascending around 1000 times short of the example above.
I think the issue would eventually become moot. Who would use a rigged currency when it's pegged to one that is un-rigged, anonymously private and globally ubiquitous? However, should precious metals become the standard for a store of wealth, this will also reduce the number of bitcoins needed, which will then decrease their relative value.
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