Consider the entire generation of 1970s and 1980s Betamax footage that is basically lost today.
When format wars snap to one side: VHS vs Betamax, HD-DVD vs BluRay, Flash vs HTML5, QuickTime vs DivX… The losing side basically loses a ton of footage.
For the most part, we know how to play Flash right now and mostly how to upconvert it or otherwise archive it. That’s not true of all formats.