Signing keys can be derived from a root key that allows for rotation and revocation. Multiple keys can sign the same content.

Now you have to securely deliver those keys to the cameras and people have to keep them up to date. With smartphones it's a bit easier because that can just be pushed to the phone automatically but for news orgs and other professional outfits their camera's aren't internet connected. So then you have a weird mishmash of deciding if an out of date key is being used because it's been cracked/stolen or if the NBC stringer just didn't update their camera before heading to the event.