"Your save is incompatible to the current version of the game."
The best part of JSON or keyed data is less profile conversions for one new key that shifts everything by a set position. Keys can be added and removed and you don't break the entire game save.
Binary blobs for game formats/saves/states is still a massive problem in gamedev. They have given many people a bad time when the profile conversion or state conversion fails and you end up with borked states. Almost impossible for that to happen with a keyed format like JSON. Getting studios to use it, especially early in dev to work with server/mobile dev was a massive challenge.