I) There will be a "danger window" where the fact that audio-visual evidence is meaningless has not yet fully registered with too many people.
II) It's perfectly possible that audio-visual lies are just inherently more convincing to the human psyche than written or spoken lies. We know instinctively that we can not trust what others say blindly, but we do instinctively trust our own eyes and ears. Note the etymology there: Blind trust is trust without seeing. When you see you don't need trust anymore.