It won't ever replace FIAT.
Fiat can scale infinitely because it is centralized. There are zero bottlenecks with FIAT.
If some magic happens and Bitcoin replaces FIAT however, it would be same as FIAT without any physical bank notes...
Bitcoin supply is limited and it is cornered by a few people. It would be total chaos and the governments would fall and the next world war would begin.
good point. bitcoin is limited, this can not be changed by anyone yet, the government either. So fiat is needed still
Bitcoin at the current protocol is limited yes, but it is wrong to say that it cannot be changed by anyone since Bitcoin had been modified for improvement several times. Remember it is only a code that is programmed by humans and the program can be altered same goes with Bitcoin but of course, it needs consensus to apply any changes.
If Bitcoin replaces fiat, I can say it is a disaster since the world economy will fall under the hand of very few people (whales). Early adopters will rise in power, more powerful and influential than a president because they holds a huge percentage of the world's wealth.