I think (my!) people don’t actually understand the difference between the grey (reply to post) and the blue (reply to topic) button.
So if anything were to pop up, rather than ‘consider replying to several topics’ (“Why? What am I doing? Isn’t that what I’m doing?”) the pop up might be explanatory: “By using the grey button you are replying to just one person. If you meant to reply to the entire topic, you can use the blue button.”