I think you make a valid point. I also think boiling the middle class down to some percentile range of income leaves us without any reason for the term itself. So maybe the percentile ranges alone aren't actually saying the same thing as "middle class."

But if someone wants to stand in front of a slide deck and explain why people who think the middle class is shrinking for the worse are wrong, I think they should either explain why that definition of the middle class is wrong, or explain why that particular definition of the middle class is not shrinking for the worse. Otherwise they're just talking past each other.