In my opinion of course, fiat is necessary at this moment, because not all people in the world can access cryptocurrency. But if you talk about future I think cryptocurrency can replace fiat, not near future of course, but someday that thing will happen sooner or later. Cause fiat replace gold in the past why cryptocurrency cant, right?
Fiat currencies definitely aren't necessary
Like for example I want to buy a single cupcake on the street worth of 1 usd but need pay 1 usd more for the fee? Or receive my paycheck this afternoon with bitcoin and wake up the next day just to find out my money decrease from 5k usd to 4k cause market volatility? At this moment I think we still need fiat money. Until there some cryptocurrency that solves the problem about confirmation time, fee, and stable price.
don't consider me serious though im not an expert