Yeah, the plain text of the database backup isn't bad, and is certainly one of the minimum things to go ahead and include. But its also a bit "phpBB-specific" in that the relationships between the tables, and the data contained in those tables, might not be so obvious to something that isn't phpBB.
One idea I explored -- but still didn't seem to be sufficient -- was to try and leverage the XML output that phpBB's RSS feed (
/app.php/feed) generates. That gets us a lot closer to "ideal text archive" in my mind, because the data is already "composed" into more of a complete "message".
But it seems like it needs even more work than that; giving a better "all messages in a specific topic" relationship, as well as perhaps rendering the smilies and BBCode references as HTML or some other future-proof way, etc.
No, phpMyAdmin "export to XML" doesn't help, and is no different than the standard plain text database export. That's "XML description of the database tables." What seems like would be more desirable for this archiving purpose is something closer to "XML description of messages and topics".