>It just makes attribution of the origin easier and creates a traceable chain of trust.

This kills anonymous sources, maybe literally under hostile regimes.

If it's signed by the equipment that originally recorded it (you're smuggling evidence of crimes out), you may not be able to verify it anyway.

It would be signed by the journalists or news agencies verifying their report.

Individual sources can still be anonymous.

So no video editing then?

For my master's thesis, I designed/built a prototype that would track video modifications and add them to a signed report like this. Maybe it's finally relevant: http://www.dissertations.wsu.edu/Thesis/Spring2015/E_Houle_0...

Maybe with a chain like with SSL certificates & authorities? Or maybe there could be a reference database with all the versions signed by a given authority so that you could verify it's an approved version? Why not go even further and show the same kind of alerts / block content if the source / signature can't be trusted?

Not without losing the signature. But if you take a clip from a larger video you can (and should) refer back to the source.