Your digital photos, letters, books, articles, documents, messages, etc. have no existence unless you pay attention to their preservation – or without electricity, for that matter. (You can’t really hold Zeros and Ones, Nothingness if you will, in your hands, let alone bequeath Nothingness to your heirs without taking serious steps to preserve and authenticate the data.)
See, e.g. from Moritz Law Library at The Ohio State University for what PURLs are:
Research Tip: What is “Permanent” Online?
Clearly, multiple print copies of your magnum opus, in libraries around the world, might be a good backup system, even if you support the Internet Archive and other preservation efforts like UELMA (the Uniform Electronic Legal Materials Act).