I feel like joining this discussion is pointless, but I'll give it ago anyway...
Money is what people collectively choose to believe in. It happens that bitcoin is the best type of money ever invented.
Money is not a belief. Money is an economic resource that provides utility to humans. The degree of that utility is then compared to the degree of utility of other economic resources in order for people to exchange those resources.
I think this person might mean that fiat currencies are based on belief systems, at least since the 70s when they were no longer backed by anything. Which is sort of true, but not entirely.
Try cashing in a bank note to your national bank, they won't have anything of value to offer you. This much is true. They owe you X worth of Y, but since de-pegging from Gold, that X is no longer worth Gold (or anything) in comparison, there is no resource anymore. But I imagine someone already pointed this out to you. Of course proponents of fiat currencies will point out that these monies are backed by the Government itself, so is even better than being redeemable for Gold, but with this they simply mean that same Government can print more money (inflate the currency) to stabilise a weak economy, and most importantly will commit to doing so, as opposed to allowing the currency, and therefore economy, to fail. So in this sense, by your own definition, this money as an economic resource is worth something because the Government says it's worth something, and they'll make sure it continues to be worth something. Or at least they will try their best. Some succeed, others fail. There's a good reason why Gold in the 70s increased from $35 to $800 since the de-pegging event, because many realised that fiat currencies were no longer backed by a popular resource, backed only by those who produce it, which set a dangerous precedent.
Your argument should therefore really be based
not on the idea that a medium of exchange like Bitcoin isn't money, but that like other monies (hyper-inflationary failing economies), that it's worthless or otherwise unnecessary. But the issue here is you have to rule out some utility as being useful, namely unrestricted international payments and ability to open an account without needing authorisation or access from a central party. In countries with stable currencies it's easy to consider this as worthless or unnecessary for sure, but in countries that lack reliable monetary policies, a lot more difficult. Try telling someone who has a hyper-inflationary currency, and doesn't have access to dollars, that Bitcoin is fundamentally useless. Of course you could argue that Bitcoin isn't a good investment, due to the current volatility, but not that the most reliable or accessible medium of exchange for someone is useless. I'll ignore the inflationary properties and ownership abilities because these are monetary policies, as opposed to utilities.
So the real question becomes, does there
need to be an international currency that is accessible to everyone regardless of geography, status, income, etc? Should countries have the ability to reduce or remove their dependency on political forms of monies such as the dollar or the euro that only leads to power being more centralised to certain nations? Obviously the Western world would generally argue no, and would rather have hyper-dependency on their own currencies, but others that live in countries that are dependant on the dollar, the euro, the pound or the rubble, but don't always have access to it (or limited access), would no doubt argue differently, as they don't have much of a choice in the matter.
To argue Bitcoin has no utility is therefore simply just baseless and no doubt comes from a position of privilege and ignorance - that of having access to a stable currency. You're better off arguing that as a utility, it's not worth $300 billion, or $1 trillion, etc, despite global debt curently being $300 trillion, or Gold being worth $11 trillion. The fact that fiat currencies have developed into a failed experiment since the 1970s, because it became dependant on humans to determine their value, is somewhat beside the point.
Bitcoin's immutable monetary policy simply rules out previous failures in developing reliable currencies. The only similarity is that adoption for new forms of currencies is slow, many remain skeptical, others simply consider them fraudulent until proven otherwise, such as when bank notes were introduced in the 17th century. Not many wanted "IOUs" back then, they wanted actual Gold or Silver, not a promise of it, but after realising they could redeem their bank note for Gold the "proof of concept" became clear, bank notes became more broadly adopted, even traded between different parties thereafter as we see today.
I was also a complete skeptic to begin with. The first thing I did when I bought Bitcoin was to purchase something online, in order to confirm it as a medium of exchange. Just like 17th century traders, I wasn't going to simply "take someone's word for it" that this new form of money was a medium of exchange unless I tested the theory out myself. So I exchanged it with another economic resource
