Self-Archiving and Preservation
People from the library community worry that archived eprints may not continue to exist or to be accessible in perpetuum on-line.
This worry is not really about self-archiving per se, but about the online medium itself. As such, it needs to be directed toward the primary database in question, which is the toll-access refereed journal literature, currently in the hands of publishers and libraries, and most of it already in both paper and digital form. If you are worried about the preservation of the online version, it is to its publishers and subscribing/licensing librarians that your worry needs to be addressed. The preprints and postprints that are being self-archived by their authors in their institutional eprint archives today are intended to maximize impact by providing immediate open access; they are merely open-access supplements to that toll-based primary literature at this time, not substitutes for it.