I think of many reasons or the possibilities of Bitcoin replacing Fiat Currency and I see the following flaws in which makes the chance of it smaller.

1. Transaction Time - When you are in a hurry or inside a line of the store by paying Bitcoin you are both wasting your time and the store's time as it will take longer compared to a fiat currency which is instant.
2. Transaction Fee - Also by using Bitcoin you are also adding additional cost over your purchase price in which is needed in order to complete the transferring of money,
3. Adaptability for Older people or for people who are not technically inclined - It may take some time for them to adapt something that is not useful for them.

Well this reasons might be debatable but this are the obvious hindrances that Bitcoin will face if it ever want to replace Fiat currency.