My personal hunch is that this will end up leading to a situation in which presenters do a cryptographic handshake that works to verify and prove authenticity. This isn't a new idea, and it has some very obvious drawbacks, but I don't see much of a way around the issue. The handshake could work great for something like official news releases, but for other instances that might come up in court, say, dash cam footage of an accident, it seems to me that the legal system is going to face some serious issues as these programs progress.

Looking forward to a future where all a politician’s quotes are on a blockchain, signed by their private key, and they chose to do so voluntarily out of fear of deep fakes.

Will remove all the useless “I didn’t say that”

It will end up back where we started, "it's not on the Blockchain so I didn't say it", while making racist remarks with friends.

"We verified the validity of our source's signature and vouch for it's authenticity"

- Journalist from a respected newsroom, when they choose to keep the source confidential. So it won't be better or worse than it is now: all about transitive reputation.

The unspoken threat to further denials is the risk that the source may go public, or if unauthorized, doxxing the person who recorded the video (assuming signatures have nonrepudiation)

That's a start! I think their video appearances should be like a car in NASCAR with permanently displayed logos superimposed from all the interests that have funded their rise.

Why would this require a blockchain?

Do politicians understand blockchains?

Rename it as “verified speech transcripts.” I don’t need to understand video codecs to watch Youtube.

Neither do we[1], so it's sort of a mixed bag.

1. True for an overwhelming majority of the body politic.