You seem to be confusing two things - system working and system being un-exploitable. Example - it's pretty easy to shoplift in any of the retail shops. In fact, shoplifting happens all the time. Still, nobody would claim retail model doesn't work because people would just come and shoplift everything and nobody would pay and the retailer would go bust. What actually happens is that vast majority of people do not shoplift, and the retail system works and produces massive profits.

> but cryptocurrencies have the advantage of being completely faceless and totally unaccountable

Advantage, but also disadvantage. If a bank cheats you, you have options to complain to regulators, to the police, to the FBI, to you Congress representative finally. It won't work in all cases, but it's an option that's available to you. If a crypto scammer cheats you, well, nobody is accountable. For some people, it's a big issue, because it greatly increases the friction in transactions.