There is no need to try and make justifications as to why cash is bad, or why it should be replaced by BTC, which it won't. BTC is more of a second option to fiat currencies, so it won't replace fiat, BTC users don't even predict a replacement of fiat by BTC.

Instead of trying to justify why fiat is bad, i would try to focus on the justification for which BTC is great as a currency, many BTC users use fiat because they don't have any choice, so their second currency is comfortably BTC, which gives them a choice to control things themselves, no third party, no censorship or possibility of confiscation, just the owner of the funds controlling the usage. People who use centralized exchanges are using BTC like fiat and the banks, BTC ought to be used through self custody, with trading through a decentralized exchange.