Everything these days will get digital the world is really changing.
Fiat and bitcoin have it's own pros and cons.
Fiat dominates physical transactions as it was way more better than doing bitcoin transactions physically if possible.
Bitcoin (can) dominates online money transactions since at the first place that was the intention why it was being created.
Currently both Fiat and bitcoins has their own advantages and disadvantages. but as you know we are moving towards technology and bitcoin is a technology. Fiat will be left behind very soon.
Well, for big country like USA, England, Japan and many big country, it will easy for moving towards technology to use bitcoin but not all country can do this fastly.
Developing countries will take time to go totally digital, as majority of them are not aware about the bitcoin yet and they are very convenient in using paper money.