I understand the technical side of the suggestion. The social and practical side is inevitably flawed.
You need some sort of global registry of public keys. Not only does each registrar have to be trusted but you also need to both trust every single real person to protect and not misuse their keys.
Leaving aside the complete practical infeasability that, even if you accomplish it, you now have a unique identifier tied to every piece of text. There will inevitably be both legal processes to identify who produce a signed work as well as data analysis approaches to deanonamize the public keys.
The end result is pretty clearly that anyone wishing to present material that purports to be human made has to forgo anonymity/pseudonymity. Claiming otherwise is like claiming we can have a secure government backdoor for encryption.