consider the UX differences between:

- just showing up as a naive user to a web service with no account, no signup. you have an ecash wallet - it works instantly

- arrive, navigate signup process pay for X time or API calls, configure it, etc.

not to mention that with ecash your money stays in you control and you hand over micropayments on demand. a prepaid API key would hand over the whole prepayment up front and you'll never get it back.

you watch 1 minute of a 2 hour video and decide it's not for you. in ecash world you paid for 1 minute. prepaid API key you paid for 2 hours.