jube if you "subscribe" to a thread by just clicking the word in the op box, it will appear below latest posts and be there be until you unsubscribe. You can then watch for updates and will be messaged when anyone adds to the thread.
If replies on older threads appeared on latest posts then at times we'd see the spam posts from spammers who trawl old threads and those private threads which are used by those who prefer to chat under the radar