From what I understand, you cannot technically entirely delete a topic. Only admin can delete PMs, yet they are still recoverable (I guess there’s just a flag in database).
Also, it would be incorrect to delete a PM thread if the other side does not want it deleted. That’s probably why the only option for you is to archive it.
Later, if they reply into your archived topic, it will be automatically moved to inbox. If it was deleted, what would happen instead?