OzJohn what matters is that films will be archived often in more than one way. What's also important is that they are archived with copies at different locations.
Yes disasters do happen and so that's taken into account. Film is just one way movies are archived now for posterity. Too much has already been lost because no-one gave a proper thought to it many years ago.
So the movie industry is now archiving on film, even wholely digital movies, and like digital there can be many copies.
Blu-Ray rip + BitTorrent = more copies of many films in HD spread across the globe in so many places that only the end of civilization could end their existence.