I'd imagine such devices would be very tightly controlled, being hard to access for civilians, and lets say limited to 1 such device per 1m people(which would also give you an idea of what sort of frequency this is supposed to be used).

The keys for every phone would be stored in a central repo, with a separate key for every phoneX every decryptor(which has its own private key). Meaning you'd need a device and the central repo to access users data.

But lets say they manage to build a bootleg version, what would be the criminal gain for them? Reading the data doesn't mean they can impersonate you, as the device wouldn't give you access to private keys used for authentication (lets even say these are deleted), only encryption.

The criminals could brick your phone and read your texts. There's only very niche cases when this would be worth it to them, like you're the subject of a highly targeted intelligence gathering op.