Just curious about Earn and the use of Bitcoin (questions are not related to Coinbase nor the announcement): What's the downside of just using fiat money? Why need to use Bitcoin (or any crypto)?
Fiat also means the email service couldn't be global, because Trusted-Third-Party-based payment processors can't operate globally owing to the global regulatory patchwork.
Contrast that with Bitcoin(Cash)-based international remittance, or Ethereum-based token sales, which are accessible to people in every country in the world.
Email is an open and global protocol, so cryptocurrency is a good fit for it.