Imagine you have a home on Airbnb , your guest sends you a payment, but its not directly to you, the payment goes through the payment rails stipulated and controlled by airbnb. This amounts to what is often 25-35% of your listening fee (payment charges, visa network, listing fee, etc, etc.) This is the middle men crypto is supposed to replace.

Only a trustworthy network can replace the current system, it must be something public, immutable and participatory , otherwise it will just centralize back to the above scenario, regardless of any intent. Essentially crypto is network code, it creates the primitives on transmission. And thus anyone (really anyone) can run a node and get a reward for supporting this security model. That's not a scam or even just pragmatic, its literally how money operates, as a incentive/disincentive mechanism.

People forget the early stock market was filled to the brim with scams, the original intent was good, but it attracted bad actors piggy backing in its lack of regulations, and it took years to clean-up, one can make an easy argument that's its still filled with fraud.

In your example crypto would only replace the visa network. Most of the fee you are playing is to Airbnb for getting you the client in the first place.