Fiat has been proven to be stable for a longer period of time since we are still using it, maybe I will believe that some investors will transfer their money in bitcoin but not to do away with fiat, they still need fiat as more than 99% in the world population are using FIAT.
Thats why an option to buy bitcoin is for those who has sapre money to do so and for a person who are well aware of internet and cryptocurrency. Still fiat is king, everyday we used it and still it it is needed in daily real life transaction. Even bitcoin is @ $10.00 fiat is still stable and important.
i think the main reason is that bitcoin is not yet so much popular and the facilities of bitcoin are not present every where as currency, therefor people are most using it for investment, but hope that in future people will start using bitcoin as currency and at that time bitcoin will be one of the most popular currency fo the world.