In general, I agree with you. The nature of internet commenting is such. There should be no expectation that the parent comment will continue to live -- the comment itself should be considered "owned" by the original poster, who has the prerogative to delete it.

But that shouldn't be an excuse for any of us to refrain from providing feedback, insight, and advice online. In fact, we should continue to do so with the understanding that our time in producing that comment/content is volunteered.

And of course, on the other side, we should strive to retain the record of the communication chain if we are the original posters. Deleting a comment is your choice, but it's probably good practice and behavior not to.