It seems that you are confusing two different concepts the value of bitcoin and the exchange rate of bitcoin. The value of bitcoin exists despite fiat currencies, whereas the price is measured in them. The former is the sum of all advantages of bitcoin that makes it more valuable than government-issued currencies. The latter is simply a measure of how bad fiat currencies against bitcoin. If all the fiat currencies become devalued, it will cause bitcoin's price to astronomical grow. If all the fiat currencies are completely gone, bitcoin will no longer have an exchange rate against fiat currencies and everything will be measured in bitcoin. However, the value of bitcoin will not change, because bitcoin will have exactly the same set of advantages.