In any case we aren't meant to have a single currency to rule them all. If Bitcoin should somehow become the only currency then we are facing much of the same problem as with the Dollar. Currencies are meant to compete with each other.
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I can’t see any logic here other than “that’s how it’s always been”. Humanity desperately needs a global, neutral, open money system. It will seem so obvious in a few years we’ll all wonder how we functioned without it.