Long term survival
Historically the survival of any particular document has been a matter of chance. Some Anglo Saxon charters survive as originals. Some older documents have survived in particularly favourable environments. For the most part, however, texts from antiquity have survived as copies several generations removed from the original and the more copies were made the greater the chance that one or more has survived.
I don't see that changing in terms of digital texts. Anything posted to Geocities, for example, is long gone unless someone copied it - archive.org doesn't seem to have got it all. If Google decided that Groups should go the same way as Wave how much of Usenet would survive?
If we are to have digital records available far into the future we need to do three things:
Have multiple archives of what is to be preserved*
Each archive needs to copy its material onto new media as old ones become obsolete
In addition to copying material archives need to translate obsolete file formats into current ones**
* What is chosen for preservation is a thorny problem. Every time an archivist decides to weed the archive their decisions will be incomprehensible to someone. I remember some years ago wandering into a 2nd-hand bookshop in Cromer and bound volumes of Nature that the county library had disposed of crammed into all sorts of corners.
** Ideally one format that can be kept current for a long time - long in an archival sense. PDF/A?