Yes and no. You're right that there's no button you can push to delete an entire account history, but wrong that there's no way to remove HN comments. We take care of deletion requests for people every day. We don't want anyone to get in trouble from anything they posted to HN, there's nearly always something we can do, and we don't send people away empty-handed. I can only think of one or two cases where we weren't able to make a user happy, and neither of those cases had to do with identifying information being left up on the site.
The reason we don't delete entire account histories wholesale is that it would gut the threads that the account had participated in, which is not fair to the users who replied, nor to readers who are trying to follow discussion. There are unfortunately a lot of ways to abuse deletion as well. Our goal is to find a good balance between the competing concerns, which definitely includes users' needs to be protected from their past posts on the site. I don't want anyone to have the impression that we don't care about that; we spend many hours on it.
The reason we don't delete entire account histories wholesale is that it would gut the threads that the account had participated in, which is not fair to the users who replied, nor to readers who are trying to follow discussion. There are unfortunately a lot of ways to abuse deletion as well. Our goal is to find a good balance between the competing concerns, which definitely includes users' needs to be protected from their past posts on the site. I don't want anyone to have the impression that we don't care about that; we spend many hours on it.