I think that the option to fully delete any student work from the Canvas environment is not available for reasons of security, liability, and academic integrity: while most faculty would use extreme caution and exercise great care to ensure that this corrective measure is enacted properly, equitably, and only under certain conditions, it still leaves open the possibility that something could go wrong that could be tantamount to, lead to, or construed as academic misconduct. For example, suppose an instructor accidentally deletes a submission for the wrong student--now that student is still responsible for resubmitting (and, of course, the resubmission might be altered, intentionally or not). If I was the student whose results were deleted accidentally and my initial score was replaced, the new score might have an undeserved substantial impact on my overall course grade. If my grade goes down with the re-take, I'm not going to be happy about that, and I'll feel that the resolution was unfair to me. If my grade improves dramatically, I reap the benefit of the second chance that my classmates did not get, which is not fair to the rest of the class. Now, suppose that an instructor's account is hacked and some or all student assessment attempts are permanently deleted--do all the students have to re-test or just the ones whose results were "conveniently" deleted? If there's no time for a major whole-class do-over, does everyone now get a 0 or full credit for the deleted assessment scores? If it's a minor quiz in a grade category with many other quizzes, then the loss of one quiz might not make much difference, but if it's the midterm exam that counts for 30% of the overall course grade, then whatever becomes of it will be a really big deal that has profound implications moving forward. With great power comes even greater responsibility, so the ability to delete student attempts/submissions is a huge request, even if we would agree to be forthright and conscientious about safeguarding its use.
Perhaps the idea of a "don't count this attempt" option is a good compromise because that's what instructors really want (my guess is that we just don't want the impact on the grade and that we really don't care if it's still present, especially if an option to hide null attempts is available)--and it maintains the paper trail for all attempts/submissions for security and academic integrity purposes. This also relieves Canvas of the liability for assessments that go missing--whether the deletion is accidental or intentional.
Gwinn