Assuming that bitcoin becomes the dominant currency of the world then things will be priced on bitcoin and then we will forget about pricing things on dollars, if dollars still exist at that point, that is an extreme scenario that I do not think it is going to happen but if that was the case that will be the way we will deal with it, about your second question that is correct, the dollar can be devalued overnight without anyone having any say on it, the FED can just print as much money as they want and devalue what you can buy with your money, but they could even issue a decree saying the current 1 dollar bills are now only worth 10 cents effectively devaluing their currency 90% overnight.