I'm going to try and make this question make as much sense as possible. Let's say we get to a point where 1 BTC = $75,000 and companies like Chevrolet accepts BTC to buy a car, would people actually trade their BTC's for fiat, when they could spend BTC? And would this make fiat a commodity? Please correct me if I'm wrong and please don't bash me for being a newb.
Bitcoin is superior to fiat in every way, and that will be demonstrated over time. In that future situation it is seems unlikely that anyone would still be converting bitcoins to fiat to purchase things. Car owners would wish to convert their cars directly to bitcoin.