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.