Deleting them on HN does not delete them from the internet. Sites like Ceddit already save/archive deleted comments from Reddit (and make some old threads readable, especially when the poster of the solution deleted his account for unrelated reasons). If HN would bring on this policy, we would get a "CN" within a day; probably even something more complete than ceddit due to the smaller scale. So, in the end, it would only make HN a worse place.