Numerous reasons why people should use Bitcoin have already been mentioned. So I think the next question would be: why should people use fiat?
I think that because of simplicity, the appeal of Fiat will be more or less popular, and one should take into account that the bitcoin user must have education.
Fiat is the currency used today. You certainly can not put aside bitcoin. Bitcoin is a novelty, so of course fiat is now superior to bitcoin. But as time progresses it will change.
I don't think bitcoin will change the world and replace fiat system, as long as the government are supporting it, this will stay in circulation. Right now, fiat is widely use by all people and bitcoin takes only a little portion of the population and take note, this bitcoin users are fiat users as well.