Fiat is relevant to cryptocurrency because fiat is the currency we are forced to use to live.
Mostly, we need fiat for buying groceries, paying taxes etc. There are a small number of places allowing crypto payments for goods and services, but they are a tiny section of the economy.
If you are investing in cryptocurrency to HODL for the future adoption on a global scale, then fiat is only a general benchmark (people will still need paying a livable wage to provide goods and services)
If you are investing to make a profit in "now" terms, then you have to equate crypto value to fiat worth, because fiat is the only option for using that profit.