Steps to reproduce & observed behaviour:

1. Create a course with two activities required for course completion.
2. Create 3 users, enroll them and let them complete one of those 2 activities.
3. Now delete the one activity they haven't completed yet.
4. After the first cron run, the criteria will be erased from db table course_completion_criteria, but the users will not be marked with a reaggregate value > 0 in the course_completions table, also their timecompleted timestamp stays NULL. They are therefore never touched again and in the course completion report shown as not completed even though the required activity is ticked as completed.

Either one of those behaviours would be expected:

1. You shouldn't be able to delete a required activity.
2. The cron task which deletes the criteria should also mark the course_completions entries for reaggregation, so they don't stay in this orphaned state of timstamp NULL and reaggregate 0 with completion entry in course_completion_crit_compl for the remaining required activity.

In the underlying case the customer was scared to unlock completion settings and considered it as less dangerous to just delete the activity, which raises no warning whatsoever.