How can I save my old, important Whatsapp chat messages from automatic deletion when export doesn't go all the way back?
What I ended up doing is loading a chat on http://web.whatsapp.com and hit PgUp until I got to the oldest available post. It was seriously slow to load the whole chat. I could load the whole thing in the app in one or two minutes of swiping. In the Whatsapp Web, though, it took many hours. I had to leave a process sending PgUp keypresses to the browser window in a loop for the whole night. In the morning, then, I opened up the web-inspector, found the &lt;div&gt; that held all the chat's contents, right-clicked it, and selected "Copy Outer HTML". I pasted in a file, and that's it. That was in Firefox. I did the same thing in Chromium, just because, and found that Chromium does not copy the whole thing. It truncates :(. To get the whole contents of the tag, I had to select "Edit as HTML", put the text cursor inside the text field, hit Ctrl+a, and copy and paste to a file.

That covers the text, and blurred out previews of images (encoded in base64 in the HTML). The media (pictures, audios and videos) are accessible in the local backup unencrypted. I copied that directory elsewhere lest Whatsapp decided to overwrite it with a backup where it deleted important media.

That's apparently the easiest way to export a chat in Whatsapp. Maybe later, I'll provide instructions on how to cleanup what was pasted into something easier to read. The HTML is seriously ugly, though. There are no good class identifiers to easily restructure it to something more readable; they're all useless randomly generated  strings.