I think the main advantage of Fiat is that it is easy to spend it, that's it.
It is hard to sell an investment to pay for a coffee, but with fiat you don't have that issue.
It's the same with bitcoin. Unless you use lightning, then it is not as simple to do transactions. But that's fine, because you can store your savings in bitcoin and spend with lightning, or Fiat if you need to.
The key is to save in bitcoin instead of fiat.
The fact that we should all invest our money to bitcoin cannot be overemphasized. The decentralized digital currency is more guaranteed than the centralized system, which is often times controlled by the governments. A people driven currency like Bitcoin can hardly collapse but we've seen different cases where centralized systems get broken down after the country's economy is collapsed. This is why I will always choose bitcoin above fiat. However, when we talk about ease of physical trading, fiat will always go first, considering that it is more widely accepted than the digital currencies.