There are many competing definitions of the middle class. Instead of sticking to definitions from centuries ago, it's better to consider how the word is used to

There are many competing definitions of the middle class. Instead of sticking to definitions from centuries ago, it's better to consider how the word is used today. And according to the common usage, ~50% of people in developed countries are middle class. The line between the working class and the middle class is difficult to define. You could describe the middle class in terms of having the extra income for some luxuries, owning your home, having higher education, having the social capital from growing up in a family with educated parents, having a white-collar job, and so on. Those with most of these attributes are middle class, while those who lack most of them are working class. What you call the structure breaking down somewhat I call a massive expansion of education and wealth that defined the 20th century, particularly in Western Europe. Communism was defeated by turning half of the working class into middle class and improving the quality of life for the rest as well. As people now had a vested interest in the system, they no longer wanted to overthrow it.