Requiring proof of authenticity is a niche use case, it doesn't seem realistic that this alone will be sufficient to ensure that most cameras in the world gain extreme security to prevent tampering by users, rogue employees in manufacturing companies, etc, etc. Also, we haven't ever been able to make perfectly secure software - an exploit that allows a key to be extracted from the camera is likely, and camera buyers won't care.

A key is just data. Even if the key can't be extracted from common cameras, there's no magic that can prevent from someone making a device with a known key (or falsely register to the magic worldwide-trustworthy central registry that USA, China and Russia trusts but aren't able to influence to get keys for manufacturing fake news) that a particular camera has been made), one that can be used to sign data outside the camera context to make it appear that it was "securely" seen by a legitimate camera.