Why is the American middle class dying? Justify your answer.

According to the Pew Research Center, from 2003 to 2014, the American middle class shrank in 203 of the 229 metropolitan areas it studied (see the link below). In fact, the middle class no longer represents the majority in the U.S. for the first time in 40 years.
Economists believe that the reason the middle class is shrinking is that middle-class jobs are disappearing. Many of the jobs that are expected to decline in the next several years provide middle-class incomes (see the NBC News article below). Middle-class jobs, currently defined as those paying $14-$21 per hour, are largely being replaced by technology. Many of the middle-class jobs that have disappeared in the last several decades because they have been replaced by technology include bank tellers, bookkeeping clerks, typists, and travel agents. In addition, as reported in the Huffington Post (see the link below), membership in unions has declined over the last 40 years, causing a drop in wages for non-union workers as well. When unions are strong, they cause a rise in wages for private sector employees in unionized industries. 
Over the next several years, lower-income jobs (such as those in fast food service) are projected to increase more quickly than middle-class jobs. In addition, upper-income jobs, which often involve technological, research, or management skills, are expected to increase more quickly than middle-class jobs. However, higher-income jobs often require skills that middle-class people may not have or may need retraining or additional education to qualify for. 
https://www.huffpost.com/entry/decline-of-unions-non-union-workers_n_57c4d789e4b09cd22d92489b

https://www.pewsocialtrends.org/2016/05/11/americas-shrinking-middle-class-a-close-look-at-changes-within-metropolitan-areas/