That key will get leaked. A key that has to go into every phone, even if done at the manufacturer and onto the TPM chip, will get out.
Also even if it doesn't get leaked directly, the security of TPM chips is not absolute. Secrets from them can theoretically be extracted given an attacker with sufficient means and motivation. Normally nothing that's on a typical TPM chip would warrant a project of that magnitude, but a widely used private key can change that equation.
Plus a TPM chip doesn't really have means to tell the phone isn't being lied to. You could swap out the actual phone camera hardware and sensors for a custom board that feeds the entire phone camera data of your choosing and it would be none-the-wiser.
> That key will get leaked.
Maybe? But biometric passports, chip-and-pin payment cards and SIM cards seem to do reasonably well. And Apple can always push out a mandatory software update that rotates the key, if they need to.
> You could swap out the actual phone camera hardware and sensors for a custom board that feeds the entire phone camera data of your choosing and it would be none-the-wiser.
Apple's 'TrueDepth' cameras are serialised and paired with the rest of the device. The touch ID sensors were before that too.
I don't know the precise details, but reports from people trying to repair devices independently of Apple are that the phone is very much the wiser.
e.g. https://support.apple.com/en-gb/120567 https://www.reddit.com/r/iphonehelp/comments/1dl38kq/iphone_...
> Apple's 'TrueDepth' cameras are serialised and paired with the rest of the device. The touch ID sensors were before that too.
That prevents trying to swap the module, but doesn't prevent swapping out the sensor on the module itself.