You speak as if income is static.
Anyone can raise their income anytime they want so anyone can increase their mobility.
Income of a population can be static even if people can go up or down the socioeconomic ladder.
For example, the median income of an uneducated white male has fallen after inflation over the past 40 years. That doesn't mean that one specific uneducated white male in 1968 makes less today than he did 46 years ago. What it means is if you took all the incomes of all the uneducated white males in 1968 and compared the median to the median income of all the uneducated white males in 2014 adjusted for inflation, it would be lower today than it was two generations ago.
To understand how everyone is progressing, we look at populations, not individuals.
No one needs to understand how everyone is progressing, because everyone is not anyone real. Uneducated White males make less today for the simple reason that there is little demand for uneducated White males in a highly technical work environment. People are individuals, they work, eat, live, hurt, and die as individuals, and not as any mass.
The fact is that individuals continually move up the economic ladder, while others move down the same ladder. That means that no one is stuck in any economic strata. Most of us, when we leave high school and start out on our own, start at the bottom and begin to work up the economic scale. Some progress farther than others, some never get off the bottom, and some fall back. Some get struck by lightening, others win lotteries. Some get sick, some become drunks and druggies, and some die. That is life. It will never be fair, but it is well worth the trip.
One serious problem that we have in our economy today, is the lack of entrepreneurs. These are the new business entries that keep existing businesses hopping to stave off the competition. New business starts are at an all time low, and that is because government regulation and red tape is becomming too difficult to sort through and too expensive for those with limited capital to overcome.