His argument was that if banks disappear, there's still some guarantee that we somehow get some of our money back, as its supported by the government.
And if your fiat currency collapses? What use is getting your money back from the government if it is only worth 1% of what it was worth when you deposited it? There are many
many examples of fiat currencies rapidly devaluing so as to be almost worthless. There are plenty of such examples going on around the world right now.
But demand for bitcoin just disappear one day and the value would vanish with it too.
How is that different to any other asset in existence? If the demand for gold disappears, then the value of gold plummets.
He says that just based on tech we cannot really move over, because trust is more important and fiat has earned the trust of the world as of now.
Fiat is printed at will by centralized governments to pay their own debts and stealth tax their citizens. I don't trust it at all.
Why would someone want to leave that convenience and want to move over?
Security, privacy, censorship resistance. You are entirely dependent on the underlying bank or other fiat payment processor for the security of your coins. If they suffer a major hack, go bankrupt, etc., then there is good chance you lose everything. You also have zero privacy from the fiat payment processor. They all log everything you buy, how much money you have coming in, how much you have going out, absolutely everything that you do, and share that information with a whole variety of third parties in order to profile and monitor you. And then you can only make payments as long as the fiat payment processor allows you do. Did you buy something they don't want you to? Did you transfer money to someone they don't like? Maybe you even posted something on social media which was critical of your government? Oh dear, your account is frozen, you can't make any payments, and you can't withdraw your money.
This censorship is not some crazy conspiracy theory - it is happening right now around the world:
https://bitcointalksearch.org/topic/recent-events-should-make-everyone-withdraw-all-their-coins-to-their-own-wallets-5387401. Bitcoin solves all of this.