Photographic evidence has been subject to manipulation before computers were even a thing, more so after Photoshop became widely available. There has always been forensics for that, which will continue to evolve.
I think the issue with trust is rooted elsewhere - in social relations, politics, and not in AI generated content.
It has, but it used to take a lot more skill to manipulate a photo than to take a photo, and convincing video manipulation was even harder. I'm also skeptical that forensics will be able to keep up, because of the basic principle of antagonistic training -- any technique forensics can use can be applied back into improving the pipeline that generates the image, defeating the forensic tool. That certainly wasn't the case in the 20th century.
I think the issue with trust is rooted elsewhere - in social relations, politics, and not in AI generated content.