Thingking that way even you dont know bitcoin that much is not really understandable well people talk about this way because they tend to criticize evrything even they dont realy know much about something. Nothing could replace bitcoin because it has a headstart of a years to grow than new cryptocurrencies.
Bitcoin cant replace regular currencies, because it is not a real currency and even lacks everything that is required of a real currency. It highly volatile, but real currencies are a near stable, and a little bit volatile (they maintain a stable price for a long time). Bitcoin is decentralized, but real currencies are Centralized. Bitcoin is not conventional, but real currencies are conventional. With all this, there is no way bitcoin can replace Fiat.
Well i believe only god knows about this kind of thing maybe we can only predict but we really dont know what would happen next.we can never tell what will be the next status of bitcoin because it is not really predictable.everytime the price may change that is why there is a big possibility that another coin will surpass bitcoin but maybe it will take a long time.