I think the publishing media that predate the web all had the same problem; posters get torn down, newspapers burn, even stone carvings weather. Hardly a new, or possibly even unnecessary phenomenon.
Now you write an article about a poster, and instead of a photo, there is an embedded instagram photo from the artist, who then removed their account, and the content is missing. Or a hotlink to the authors website, and it's missing there too.
Yes, if the book was destroyed too, all info about the poster was lost, but atleast the book didn't say "go to the corner of X and Y street, and hope it wasn't removed or destroyed by the weather".
A photo of an inscription is not the same as the inscription itself --- detail is lost in any translation.
For digital works, it is possible to faithfully reconstruct an original with full fidelity (if necessary, embed an emulated environment of the host, server, or network originally provisioning the work). But copyright claims make this a legal suicide maneuver, at least for any entity capable of being sued, or being sued effectively.
Note that numerous previous archivists have in fact been pirates or copyists, sometines under pre- or non-copyright regimes, but very often in direct rejection of copyright.