In both cases, the reference is only as good as the maintainer. If you
want reliable links then only link to reliable sources. And why not have
electronic libraries that archive the data of popular URLs? If someone
wasn't saving all the books you would have the same problem.
I have my doubts that everything published electronicly is worth saving.
However, that doesn't mean that it isn't worth being published. Take for
example mailing lists and newsgroups. Some of them are archived forever
and ever and some are not. But even in the later case people are still
happy to create followups and reference articles are soon drop into oblivion.
--sanders