This is why you should archive the packages along with the source for your document. This way you'll maximize the chances of being able to regenerate it at some point in the future.
Why wouldn't LaTeX also require the OS to be archived for equivalent results? Although LaTeX often uses it's own bitmap based font system it does require X and supporting infrastructure.
Because unlike Word, LaTeX can easily be rebuilt from source. And it should work on future versions of the OS. Though, of course, there's no guarantee of that either. So might be best to archive the OS too, just in case.
