Good point, well taken. However, that still leaves the mystery. I've looked at a couple dozen new and old threads, and reply icons used to appear more frequently, though not as often as I mistakenly remember, I must admit. One possible explanation that occurred to me is that it might be related to the icons themselves (unique icons versus generic), but I have found every combination once. I have taken to always quoting, if directing a comment to a specific poster, so there will be no misunderstanding.
By the way, I notice your reply above has no reply icon.