I think it's practically impossible for such a system to be globally trustworthy due to the practical inevitability of "but improper storage of private keys lea

I think it's practically impossible for such a system to be globally trustworthy due to the practical inevitability of "but improper storage of private keys lead to vulnerabilities" scenarios. People will expect or require that chain of custody only if all or at least the vast majority of the content they want would have that chain of custody. Photo/video content will have that chain of custody only if all or almost all of devices recording that content will support it - including all the cheapest mass-produced devices in reasonably widespread use anywhere in the world. And that chain of custody provides the benefit only if literally 100% of these manufacturers have their private keys secure 100% of the time, which is simply not happening; at least one such key will leak, if not unintentionally then intentionally for some intelligence agency who wants to fake content. And what do you do once you see a leak of the private keys used for signing the certificates for the private keys securely embedded in (for example) all of 2029 Huawei smartphones, which could be like 200 million phones? The users won't replace their phones just because of that, and you'll have all these users making content - so everyone will have to choose to either auto-block and discard everything from all those 200 million users, or permit content with a potentially fake chain of custody; and I'm totally certain that most people will prefer the latter.