Things are not so simple, in theory a better product should replace an outdated product every time but that doesn't always happen, you need to understand that there is government intervention and they are not going to let bitcoin to replace fiat, they obtain a lot of power and money out of the right they have to print their own currency and they are not going to give up that right without a fight, it is possible that in the future bitcoin is subject to very heavy regulation or even banned, that is not going to destroy bitcoin but that is going to severally limit its adoption.