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.
Your reasoning is sound but that is precisely why fiat currencies are not going to disappear, governments love to tax you without having to pass a law for that to happen, that is why the governments love printing money so much, they get all the benefits of taxation, they can tax your wealth and not only your income, and they can do it in a way that not many people realize what is happening, so governments need fiat currency and if they need it they are going to keep pushing it so people keep using it.