I don’t want to get into a contest debating which school is better (they are more or less equals), but here are some counterpoints to your assertions.
- It's been some time, but NU broke the top 10 in 1997 (http://www.adminplan.northwestern.edu/ir/data-book/v48/1.01-usnews-undergrad-rankings.pdf)
- There are a number of departments for which NU is ranked higher thank Duke. Below is a sample: Chemistry (http://www.shanghairanking.com/SubjectChemistry2015.html) Psychology (http://grad-schools.usnews.rankingsandreviews.com/best-graduate-schools/top-humanities-schools/psychology-rankings) Sociology (http://grad-schools.usnews.rankingsandreviews.com/best-graduate-schools/top-humanities-schools/sociology-rankings)
- For economics, there's probably only a slight difference at the undergraduate level, but the gap is bigger at the graduate level. Read through what the econ PhDs think about the two programs: http://www.econjobrumors.com/topic/duke-vs-northwestern
- There are other dimensions on which NU beats Duke (e.g., endowment), and others for which Duke beats NU
Again, I think both equally excellent schools.