This is kinda going in the direction of an individual's right to their own work versus some kind of community rights, which, as I said, should be clearly stated in the TOS.

If someone rage quits, that's their decision. If they no longer feel that their content is safe in the hands of that community, that's also their decision. Of course, we can probably limit what you're talking about by managing between being able to delete individual posts or entire threads. There's always also this lovely thing called compromise, such as if someone is leaving a forum that won't allow them to delete their story or art threads, at least they should be able to ask (or tell) a mod to remove it for them.

The main point here, I think, is:
It's not about "keeping a record". It's about ownage and usage rights.

Afterthought: I admit that I don't know much about site handling, but don't most mods keep some kind of a backup system for important threads or keep some kind of loose record of site activity anyway?