The shift from one medium of exchange to another is just a way of improving and looking for what's better. From the ancient trade by barter, it limitation brought about the fiat and then it went on to an era where most of the transaction is not digital with fiat still being partly used for physical transactions that doesn't involve a huge amount of cash.

The digital currency hasn't reached that level where it can completely put an end to the usage of the fiat and certainly, the digital divide in the world wouldn't let that happen at least at the moment. The physical currency is still the most simple currency that the different class in the society can easily make use of regardless of technological knowledge or whatsoever. There's an obvious shift but there's not a replacement of the fiat.