there is some chance that one of the currencies will cease to exist
Nitpick: I don’t think this is much beyond epsilon. Even if a currency ‘ceases to exist’, the currency still exists in whatever physical embodiment. There is still an exchange rate from Saddam’s dinars to US dollars: it’s however many pieces of old paper $1 will buy you on eBay. The exchange rates for currencies that cease to exist can even go up over time (how much would a American Revolution-era shinplaster cost ya?).
Nitpick: I don’t think this is much beyond epsilon. Even if a currency ‘ceases to exist’, the currency still exists in whatever physical embodiment. There is still an exchange rate from Saddam’s dinars to US dollars: it’s however many pieces of old paper $1 will buy you on eBay. The exchange rates for currencies that cease to exist can even go up over time (how much would a American Revolution-era shinplaster cost ya?).