People have already tried this with C2PA, and it's not a perfect system. The main problem with these systems is that you risk certifying fake media, which is worse than uncertified fake and real media.

https://news.ycombinator.com/item?id=40313769

I do wish more people used basic watermarks on their media regardless - as in sloppy ffmpeg text. People are more likely to repurpose other people's older media than generate new media, and old time-y watermarks make that just slightly harder. Maybe a free app for uploading citizen journalism media that handles the watermarking with geo- and datemarking is the way to go.