A lot of times we want Deleted User posts to stick around because, while the user may not be around, what they said might be useful and/or relevant for the conversation.
However, one thing I want to eventually add is more control over threads for the original creator – and one thing that includes is being able to delete posts in their thread. This would be able to help clean up posts where Deleted User posts aren't relevant anymore without having to delete all Deleted User posts automatically.
Being able to archive posts somewhere else is a good idea, also. I'm also planning on making it possible for thread creators to lock their own thread, which might help on the don't-resurrect-old-threads front, too. :)