Northwestern’s yield without ED would have been higher if other schools also do away ED. It’s unfair to single out a school and hypothesize what its yield “should” be when you hold other schools’ policy unchanged. Northwestern doesn’t shower admits with merit-based scholarships and its FA, while not bad, isn’t among the best.
By the way, Michigan is cheaper than many schools. That in itself helps a tons in yield against the privates. But then there are many flagships that are cheaper than Michigan OOS. I don’t know what the real point of this exercise and discussion is. It’s not like yield necessarily reflects desirability when finance gets in the way.