I define the bottom line of "middle class" as having disposable income, or equivalently, not living "paycheck-to-paycheck."

I have lived in both. Being in the working class sucks.

That said, some people in the working class can save a few months' expenses or even retirement. Usually as they get older and get more stable jobs, such as in a factory. But the toys they can afford are far smaller. At that point, there isn't really a discrete line.

If you make as much as I do...you might be middle class

Yep, and that's why politicians appeal to the "middle class."

There's also a strong cultural and psychological factor. There are people in the working class making $95,000/year. I know that sounds contradictory, but it's really not.

If you’re respected by your peers...you might be middle class

Nonsense. There are just as many working-class respected and respectable. The middle class might look down on them, but not their peers.

He theorized that as machines replaced physical workers, former manual laborers would gradually become managers and administrators instead.

He was right. I hate this trend. Most organisations have ten times the necessary managers now, and it makes actual producers far less efficient. But it's necessary with our labor-based economy. Without it, we'd have 30% unemployment. The whole system is repugnant.