If you own only one device—a mobile phone, like the majority of people in the world—and you lose it, how do you get back into your passkey-protected email account?

Most Passkey implementations by default are syncable/copyable/backupable. The iOS default is iCloud Password Manager and keys get backed up to iCloud and restored on device restore when replacing a phone (just like the rest of the users passwords they choose to store). Android has similar defaults with the Chrome Password Manager.

Backup codes, sms backup, trusted user backup. Most of these are quite insecure but better than loosing access to your digital life.