5th option: Apple (or whomever) could sell a "backup box": a simple embedded device with RAID 1 (mirroring) storage and the usual data/charging port(s). First

5th option: Apple (or whomever) could sell a "backup box": a simple embedded device with RAID 1 (mirroring) storage and the usual data/charging port(s). First time you plug an iphone into it you are asked to verify using the connected device to store backups. The user then uses the "backup box" as their regular home charger. Backups happen automatically while charging. The user doesn't worry about keys; attack surface is limited to physically local risks (no network!). People already understand how to protect a physical device: lock in in a safe, move it to a safer location, etc. Instead of trying to solve the security problem for the user, give the user tool they can understand that allow them to protect themselves.