It's really clear when you consider the outcome:
Nationally, the proportion of middle-class adults shrank to 51 percent in 2014 from 55 percent in 2000, Pew found. Upper-income adults now constitute 20 percent of the population, up from 17 percent. The lower-income share has risen to 29 percent from 28 percent.
The percentage of "middle class" adults decreased by 4%.
The percentage of "upper income" adults increased by 3%.
The percentage of "lower income" adults increased by 1%.
In other words, 3 times as many people moved from middle class to "upper income", as those that moved from middle class to "lower income".
That would appear to be a positive trend, overall.
I think what the issue is what David Stockman calls the breadwinner jobs. Those are jobs that pay above $25. This number has been stagnate for years. We still have less of these jobs than before the GFC hit. The last time I looked at his site, it was something like 78M breadwinner jobs. You don't necessarily need one of those jobs to be in the middle class but it sure does help. A couple both earning $15 / hour FTE can do fairly well in my town and even better if one of them gets health benefits.