We already couldn't trust anything a politician said! But indeed, this is the moral equivalent of how Photoshop has nearly undermined photographic evidence. That leaves video, and it's starting to succumb. Synthetic video AI politician, well, Robert Heinlein wrote _The Moon is a Harsh Mistress_ in 1966.
It is much harder to look at a photograph now and say for sure it's real. Before photoshop, there were ways of manipulating photos, but they were much harder and did not yield nearly as good results. You can photoshop people into photos they were not originally in, doing things they have never done. Even Trump is having his hands enlarged in photos.
Maybe you're speaking specifically about "evidence" in the legal sense. I can't speak to that. But there are countless examples where you can't (and shouldn't) believe what you see, because it's not real.
Certainly it raises the bar for skepticism, but I can think of relatively few cases where a photoshopped image has driven a news cycle/key event because someone famous/important believed it to be real.
Most doctored images/videos get snuffed out by the media very quickly if they grow viral.
So because you can think of relatively few cases where key figures were duped, it therefore must not be a problem? That hypothesis fails to account for the rest of humanity.
I have a friend who believes in aliens. They showed me a video that someone had put together of "footage". They were seriously showing it to me as evidence, to convince me that they are real, based on this video. I was shocked they were so serious - and having a hard time containing my laughter.
I later found a number of other videos debunking the videos and clips that video was based off of - but my friend still believes in aliens, based on those videos. And this is someone I regularly have "intelligent" conversations with.
This change, the ability to put words in someones mouth, is the next photoshop. And it WILL have consequences to it, good and bad. And we are not prepared for that, as a society - we haven't gotten over photoshop yet.
