All you say here is 100% correct.
But what you have left out of the formula is competition.
Before the 30's...if you were GREAT at what you did and the guy next to you sucked ass.....there was very little you can do becuase your employer knew you had no where else to use yopur talent. The closest company that would want your exact talent was 500miles away.
However, since then, we find competitors right next door from each other.....and if you are great at what youy do, you are paid much better for your empoloyer doesnt want to lose you to the competition.
Andf that is why the higher level people get paid so well...
As you said in another post, it's not just what you do but when you do it. But let me question your idea here of why that's so.
First off, let's say you're an automobile worker. Here's Wikipedia's article on the auto industry in the U.S. just for general info:
Automotive industry in the United States - Wikipedia, the free encyclopedia
According to this, the auto industry began in this country in the 1890s. By the late 1920s, it was going great guns. The main change came with Ford's introduction of the assembly-line method in 1913, which allowed mass-production of cars that could be afforded by people with less money. Ford's competitors caught up with Ford over the 1910s and 1920s gradually, and the industry reached a peak in terms of sales just before the Great Depression threw it a curve ball.
Now what that means is that Ford had a virtual monopoly on lower-priced cars in the early 1920s, losing that monopoly in the late '20s. So in 1921, an assembly-line auto worker had fewer places to work at than in 1928. Your prediction would be that wages would go up between those two years, and they did somewhat, which bears out your idea being one that works -- to a point. (Which makes sense; it's simple supply-and-demand stuff.)
But then the Depression hit, and a lot of American automakers went bankrupt. The Big Three survived and a few smaller companies (like Packard and Studebaker). So in the 1950s, there were actually fewer companies making cars than in the late 1920s. Concentration wasn't much of a factor between the two; the auto industry was concentrated in and around Detroit, Michigan in both eras. (Not so much so today.) So how can the availability of alternatives account for the difference in pay and benefits between 1928 and, say, 1955? I don't see how it can.
The obvious (to me anyway) answer is that in the 1930s, the auto industry and most other manufacturing was unionized. What that meant was that, as productivity increased, the labor force was in a good bargaining position to demand a good share of the proceeds of that productivity increase.
It used to be that a receptionist that could handle 20 incoming lines with professionalism and personality was a gold mine.....Now? The recptionist is there for the voice mail overflow.....she rarely gets to show her talent as a quality receptionist. So now she is paid a basic salary as her responsibilities are much less than they used to be.
I can see this, but to me, the changes to any one job are less important than the changes to the employment picture overall. Basically, reception work has dropped from being a demanding, skill-intensive job to being a low-skill job, but there is still a spectrum of low- to high-skill jobs. My point here is that the high-skill jobs (which aren't the same as they used to be) pay less these days than high-skill jobs used to pay. That's true until you get into the very-high-skill or professional area, and even there, salaries have been held down, e.g. in the software engineer area, through creative use of government.