Yeah you though of good question but it's simple in my view. Comments downvoted and later deleted doesn't change a person any single thing, but his action has helped the platform by removing spam. As for does who gain reputation, this is decentralisation, so if we start playing censorship as many users seems to like, the future of the company is in vain. So far I see no reason where a user will delete a comment after earning a reputation because that left as a proof of his work. Another thing, you can't delete a post but only comments, everything in introduction is comment hence why can be deleted.