in the future, will there really be a need for fiat currency? Government can collect taxes in crypto, determine how to allocate it, and spend it in crypto. Ppl can receive salaries and do all commerce using crypto. Loans can be done in crypto. Fundraising can be in crypto. Sure, problems have to be worked out in terms of scalability, does it have to be completely decentralized, etc, the value has to become more stable. But theoretically, is there any reason to have fiat in the future?
It has been established that governments can now do taxation without representation by printing more money (which is a tax on everyone holding the currency), instead of trying to get a bill passed that allows it to increase the tax rate. With crypto this would be no more. All increased taxation would have to pass a vote.
From an economic standpoint, the idea that a country can stimulate its economy by adjusting interest rates is tenuous. There is just as strong an argument that lowering interest rates has caused speculation and bubbles, with calamitous results. In a crypto market economy, capital will flow from where it is needed less to where it is needed more.
The thing is that governments will be unwilling to give up their control over the economy that easily and letting its citizens just used decentralised cryptocurrencies to conduct their transactions instead.
Hence, it's very likely that fiat currencies will continue to exist, as long as the government and central banks exist. They will always want to keep a hand on the economy.
An economy purely run on cryptocurrencies without a central bank can definitely be done. However, the question is whether it would realistically happen. And right now, chances are that in the future we'll see fiat continue to exist. Even if the government decides to shift to cryptocurrencies, it will be in the form of a centralised crypto that they can adjust the supply of.