Nobody is arguing that. What people are arguing is that if copyright does not promote the progress of science and useful arts, it fails to fulfill the intent of its existence. The follow-on argument is then that indefinitely extending copyright undermines that promotion of progress. If you buy both arguments, then you see why many people find the extensions so objectionable on Constitutional grounds. I don't think many people disagree with the first argument, because it's right there in the text (which is the point I believe your parent was making), but of course lots of people disagree with the second argument, believing that longer copyright terms better promote creation.