I agree with you that bitcoin will probably will never be the main global currency, but it can replace the current internet payment systems, instead of paying with credit card over the internet that will also require you to reveal your credit card details, you can play anonymously with bitcoin.
Fiat defends citizens from them-self, if regular people will start using bitcoin on a daily basis, the chances of them to get scammed is pretty high considering all the scams that are running on the cryptocurrencies market.
Unregulated market is really dangerous for some people, and that's what fiat comes to defend.
if bitcoin has to be a global currency, I think it's quite difficult, but in this world nothing is impossible, but the process may be quite difficult because all countries in the world already have their own currency.
but if bitcoin becomes / helps in payment in the internet world it is very likely and it will be very helpful to users and maybe it will be welcomed by all people.