Fiat currencies definitely aren't necessary, there is an increasing trend towards digitization of currency, and blockchain based currencies are just the next generation of this. The problem is that Bitcoin came out, and everybody treated it as the successor to fiat currency, rather than an incremental improvement on digital currencies such as Paypal and Skrill. I think that if Bitcoin was never invented, but somebody like Paypal had implemented a blockchain technology first, then we wouldn't be in the rivalry scenario that we're in now.
I oppose to your idea. In my opinion, I think fiat currency is necessary. Let say that bitcoin or other cryptocurrency is really helpful today but we cannot change the fact that fiat is more appropriate currency to use in everyday life. It is still better if we can hold our money rather than stocking it in some devices or apps. Although, bitcoin can help us in everyday life still we cannot be sure in digital currency so basically, fiat is a need.