I'm not sure, but consider that your HN username is something you might submit when applying to YCombinator. So, it makes sense that they wouldn't let you dele

I'm not sure, but consider that your HN username is something you might submit when applying to YCombinator. So, it makes sense that they wouldn't let you delete the account; for better or for worse, you might have to explain your posting history to PG during an interview. Also, consider how annoying it would be to remove all references to a deleted account from threads--not really worth the computational power.