I think Anakin replaced the old dude because:
1: To tie the prequels & sequels together. Remember there are no ghosts in Ep I and II and III (?)
2: No one knows the old Vader and can't relate to the old man under the mask. But everyone knows the young Anakin
3: Anakin reverted to his good self. No ghosts of Sith exist, so the young Anakin (before turning to the dark side) is the ghost.
4: Lucas just couldn't not f_ck with it one more time....
