Before Photoshop, people believed in manipulated static images (as seen by all the fake UFO "photographs"). After Photoshop, it required "video evidence" to make people believe in UFOs again. Once it's possible to easily fake video material, the same will happen as with doctored static images. People will stop believing in video without additional evidence that the video is actually real.

PS: People being manipulated by propaganda is really old news. The only thing that the internet has changed is that all the village idiots who easily fall for propaganda discovered that each village has its idiot and they started to communicate and coordinate. But that's unrelated to deep fake and was already a problem before.