A Discourse to standard markdown converter would give a lot of flexibility. Category, tag, author, visibility, etc metadata could be stored as frontmatter. A custom or existing static site generator could convert the markdown files to HTML. Files could also be read locally with note-taking apps that support markdown. Discourse could also provide a markdown → Discourse migration script to allow archived sites to be converted back into forums.
The markdown approach would let Discourse promote the idea that a forum’s content is independent of Discourse the application. It would make it conceivable to think about posts created on Discourse being read 100 years in the future.
There are cases where people come together online for events that have a natural beginning and ending. For example, preparation and followup for an online seminar. Discourse could be an appropriate tool for this. By making the lifecycle explicit and providing an archive of the content, a forum that generates say 5 topics and 100 posts over 2 months could be seen as a success instead of a failure. I suspect that an explicit end date would also make some people more willing to sign up and participate in the discussion.