Not all users who work in team have professional mindset. I will request for a feature were the assigner of Task can lock deleting of task / subtask / attachment for other user. We can have a small lock icon near task / subtask / attachment which the assigner can click to enable. If the assignee edits something then trail of that edit must be recorded and shown in the window.
Please do this otherwise in emerging countries like India it will be very difficult to use.