From an application developer's perspective, the nice thing about using a passkey with a browser cookie is that you don't have to store anything sensitive. You're only guarding access to your own app with meaningless numbers. If your app doesn't store
other sensitive data, the blast radius is small. There are still denial of service attacks to worry about where an attacker can use network or compute to run up your bill.
What are we guarding when building an app that uses a cloud API that costs money? Access to more compute resources. Probably a lot more than the app itself ever uses. It raises the stakes a bit. Still, in monetary terms, you're operating a vending machine that the user puts money into.
Maybe there could be some kind of protocol and workflow to securely buy a dollar of compute time from an AI vendor?
If they send some of the money to the app developer's account, it's starting to sound like an app store or micropayments system.