Good luck watermarking plaintext and locally run models. There is no good option. If you want certainty that you are dealing with a human you lose privacy. If you want privacy you cannot know where the plain text came from unless you sign each file cryptographically. Then you only know it came from a certain source, but there is no guarantee how that source made the text. Welcome to the new world.
It’s not even about trust. It’s that I am confident I will have no clue who is a real life human being anymore soon. Autogenerated images, video, and text is practically in its infancy but already exists in the uncanny valley of being impossible to determine which is real and which is not. Imagine 5 years from now when perfectly lifelike high res video of practically anything you can imagine can be generated on the fly. Essentially the only thing I will have any certainty on is what I can witness in person. Or, if I have a circle of trust I can choose to believe content published by certain organizations or groups.
It may actually push us away from tech and back to the community, which could be good assuming we survive the transition.