A bunch of people will believe it for some amount of time, and given the very low cost of producing it, it’ll be enough. If there is one thing we learned from the recent increase in mainstream extremism fueled by social networks is that fake rumors don’t have to be especially reasonable to be effective; they just need to be generated and broadcasted at a sufficient rate (selection will do the rest of the work). One interesting question is: is one protected by freedom of speech when generating fake videos clearly intended to harm another person’s image?

I think that will be a problem of its own. e.g. In Atlanta police recently shot and killed a protester and other protesters are saying it was murder while the police are saying the protester shot first.

Right now, I hope the truth will be revealed by police releasing body camera footage. In the near term future that footage will satisfy nobody and we'll all be left wondering what really did happen.

There are videos of Nessie, Bigfoot and Aliens for decades. Are we still trusting video?!

probably only in some corner cases. There's always context and connection to real world events that cameras capture, so a fake bank robbery or a guy in two places or what have you doesn't make any sense. In a way that's already how we deal with potentially staged or doctored video evidence. You always need to figure out if a piece of evidence corresponds to the rest of your evidence.

To pick the example from the other user, if video footage of a shooting matches neither ballistics, eye witness accounts or other evidence on site it'd be very easy to spot a fake, without even technically analzying the video itself.