Generally, signing digital files is a great way to have some strong form of authenticity. There's a lot of unsigned content on the internet which makes impersonating people quite easy. We have fake news, fake comments, fake reviews, propaganda departments from intelligence agencies from hostile countries meddling with elections and public affairs, companies and lobby organizations spreading FUD, marketing companies specializing in bending public opinion, etc.
There's a lot of good reason to start insisting of people using digital signatures for everything they produce. Even without considering AI.
People obsess about what AIs can do here. But you might just make the point that at some point it starts rivaling what a determined hostile person could do manually right now. Sure, it adds scale and convenience for those with such intentions; which makes the problem more urgent than it already was. But it was pretty bad before that.
Putin did not use AI a few years ago. He simply just put some teams of writers to work to flood the internet with content intended to bend Elections this way or that way or erode public confidence.
Web of trust has been tried before of course but IMHO it deserves another chance. The current state is not sustainable when we might get AI generated content dwarfing the amount of genuine content very soon.