50 years ago this class of people was <5% of people. Today it approaches 30%. The lower class has also shrunk. So this has come at an expense to the middle class.

It isn’t that the middle class is poorer but rather are closer to the lower class than upper middle. Before the middle class was much larger and more people were similar and people compared themselves that way.

This is why housing in some markets seems insane to people in other markets. And why middle class people are locked out when living in an upper middle area.

Lines up well at parts. It isn't the full explanation.