I think another crypto will replace fiat. Not bitcoin. Bitcoin will be like Linux, that crypto will be like windows or mac. Bitcoin BTC... BCH might be that crypto.
It has certain characteristics that make it useful that fiat can't do. Like sending to another country in a second. Just like a credit or debit card didn't replace fiat.
If Bitcoin remains an obscure vessel for only a select subset of transactions, then the game theory behind the network crumbles over time as difficulty increases and block reward drops