Most of the so called middle class are just well paid members of the working class.

Small business owners and others who get much of their income via ownership rather than wages/salaries are often indirectly but strongly dependent on the salaries of the working class to stay competitive.

So really most people are in the same boat when it comes to these things. It just manifests differently.

You are right that those who start off being well paid in the working-class tend to buy capital with the differential in pay, and thus move into the middle-class upon doing so. But, as such, still realize the gains on the capital side, so the decline on the wage side is not the whole picture for them.

I don't think necessarily, it just describes of the national productivity, how much of that produce is allocated toward labor rather than capital. (wealth generated through assets vs. wages)

The middle class owning more assets than working class sure is better off. But both derive most of their income through wages rather than assets.