Apple just needs to design good UX to help these people.
Perhaps a large cardboard 'key' comes with every phone. When first setting up the phone, you scan a QR code on it to encrypt your data with the key, and then you tell the owner that of they ever lose the key to their phone, they won't be able to get in again.
People understand that - it's like losing the only key to a safe.
From time to time you require them to rescan the key to verify they haven't lost it, and if they have, let them generate a new one.