Do not trash Fiat yourself when it's actually needy, crypto and Fiat must co-exists until the government decide to go digital with Fiat themselves, and Fiat is not trash.
We must not because of the introduction of a new digital currency trash away fiat, when we are yet to see the massive adoption for the use of bitcoin as a legal tender by most countries, the fact that we are allowed to use it does not give the support of the government on it acceptance for use or approval as a legal tender, even though they also realized it as a digital currency, but they still feels its unsafe to that extent because of how such can be used for scam or fraud, while bitcoin is not competing adoption with fiat, but only Exist as a financial alternative.
From an economic standpoint, I would say cash to me becomes trash when its exchange rate gets way higher on the market and incurs higher interest rates.
At that point, Bitcoin or alternative means of exchange like gold, can become more used while cash becomes for meager necessities that don't cost much.
Cash will also tend to become trash if cryptocurrency gets accepted and regulated and gets used more often, having become a part of society and trades. It may still exist but it might become of less benefits to those in more urban communities that has developed and operate mostly on cashless economy.