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.
I think both are good in their respective ways. The convenience of fiat is not just about being easy to spend but also lies in its widespread acceptance and simplicity of use. Even if we can spend Bitcoin for a coffee, the restrictions and limitations placed on it in some regions will definitely make it harder to spend. Besides, either fiat or bitcoin, as far as we tie it to investment or savings, I believe they can not be spend in every occasion as there is a purpose they're meant for.
The key is to choose the option one can handle its risk imo.
Fiat is flawed, sure. But the way things are right now, Bitcoin is a gamble, not some magic fix.
So, pick your poison.
Life itself is a gambling. I would prefer to gamble with bitcoin than having faith in fiat and still end up losing. At least, I will be proud to have taken the risk if I win eventually.