I recently found hundreds of ghost tasks in our Asana account, which turned out to be mostly subtasks of completed tasks. I was able to clean them out (I think), but it left me wondering why they were there. I personally would consider it more intuitive if subtasks were automatically marked complete when completing the parent task — with maybe some kind of notification to make sure the user knows what happened. (“Congratulations! You just completed 15 subtasks with that click!”) … Certainly in my case at least, none of those subtasks will ever see the light of day after the parent task is marked done, so there is no reason to have them floating around.
This is similar to an existing request (Mark all subtasks complete before allowing tasks to be marked complete) but with a different angle. That request is to prevent (or disallow) completing the parent task until all the subtasks are marked complete. Which is one way to do it. Whereas I’m proposing allowing the user to complete all the subtasks by completing the parent task.