This is what's being accomplished by one amateur with a little bit of hardware. Imagine what will happen when you have professional services (legal or otherwise) making altered videos of whoever or whatever you want. Want court evidence of your wife cheating? Fake grainy hidden camera video. Want to discredit a politician? Here he is picking up a male escort and using racial slurs. These sorts of things could be used…
For instance, at least in Russia you just get an actor that looks like the politician, film them doing some bad stuff and blackmail them. Pretty simple.
We have had doctored photos/videos for as long as the mediums existed.
If anything it will saturate and you wont be able to tell what is real or not. Most people will default to "fake" and have to rely on the trustfulness of the source. Just like now. We have photoshop and there isn't mass chaos.