America became a super power on the strength of its large middle class. A large population in middle class leads to vibrancy and a market that buys and sells.
Middle class creates a vibrant market
Too much money in the hands of a few makes the economy weak and slow growing. It cannot create a vibrant market. After all how many houses can a super-rich build? One palace, or may be two in ten years. Then he will live there. Whereas regular employment is possible from a 10,000 small houses (200 yards). They need to get white washed regularly. So many taps need to be fixed. A few plumbers, carpenters are constantly busy. So are electricians. Many maids are required to help with cleaning jobs. How many will a palace employ, a hundred? This is not the same possible from 10,000 small homes.
When someone builds an additional floor, the others in the neighborhood start thinking about increasing their income. They may start working harder to take a second job or invest in an IPO of a new company. All these are socially productive. Whereas, the life of luxury of a few does not create great jobs. They only need sycophantic servants and yes-men. Plus, people become lazy once they get super rich.
Middle class improves the economy
Instead, when someone becomes middle class, they are having a better life but are not rich. They still need to work to maintain their lifestyle. As a result, they now have become more dynamic, driven to work to sustain their lifestyle. So middle class becomes like an army of ants. They are constantly doing something. This itself improves the economy and is good for the society. Whereas, the super-rich become more equal than others. They tend to distort and destroy the society by forcing privileges that are not fair.
Large companies get created serving middle class
Serving the middle classes, large companies can get created like Ford, General Motors. While companies serving the super-rich tends to be small like Ferrari, Rolls Royce etc. Even plane manufacturers would be small, if they only served the planes built for the super-rich. Boeing and Airbus have become big by serving the regular middle class people who prefer to travel in coach class.
All vibrant countries have a vibrant middle class. All decaying and lousy societies have the super-rich and the super poor. The middle class determines the destiny of a country.