It doesn't have to be. In fact I don't think it ever will be due to government protecting their currencies. However, it is worth $200 billion currently and gold is worth $8 trillion. If it just takes 10% of the current gold market, we are looking at $800 billion and 4x. If it takes something like 25% of the gold market, which still might be conservative, we are looking at $2 trillion and 10x, which is over $100k per coin... and that is without challenging fiat in any meaningful way.
And current transaction fees are roughly $2 or so:
https://bitinfocharts.com/comparison/bitcoin-transactionfees.html#1y