This is the case for the regular internet too, isn’t it? My comment lives in a cache file in your temp folder.

By definition, a cache will only contain things you've accessed (or pre-fetched to be fair to the latest javascript whizz-bangs)

Aether proposes to store its entirety on your local drive, although it is all text content, links can still be dangerous to blindly store

Depending on how everything plays out, I might migrate to a default mode where only the communities you’ve subscribed to is shared from your computer to allay these concerns. The current mode of share-all would remain available as well.

Then...Usenet? But a little nicer because it's text only (I don't recall the level of fetching Usenet has exactly, I might be wrong).

Your server of choice hosts Usenet, your client doesn’t fetch anything unless you tell it to.