Ironman thinks the decline of the middle class is demographic, explained by fewer births between 1964-84

Here.

In other words, if you have fewer people in the middle of your data set (Generation X), you'll necessarily have a slump in the middle between old rich Baby Boomers and the more numerous than Gen X Generation Y, which is young and poor, as is everyone at that age.

The middle class decline is therefore most likely to be temporary.