Here's a concrete example. There was a guy that had a PhD and worked for big pharma as a researcher all his life. The company he was working for downsized and he was left unemployed at 54. He looked for another position somewhere else, but he could not find a position at his pervious level of seniority: he had fallen off the career ladder. He was thus stuck in a strange limbo: he was capable of doing things below his level of experience, but companies were looking for someone at the right level of the progression system to fit the slot.
The worst thing about this case is that his wife had multiple sclerosis, and so his health insurance was of the utmost importance to him. This caused him incredible stress as he continued to look for work to no avail.
The problem with the modern workplace is specialization. Some people are extraordinarily specialized. The man I spoke of was highly talented, but also highly specialized. It's hard for someone to adapt at 54 to another specialization. I actually helped him pick up python, and he learned quickly, but it was clear that the man was not in his spry years. Any college kid could beat him in productivity in some code monkey role -- which is what he eventually started trying to hunt for.
I could also tell you about a guy that had a masters degree in chemical engineering working at Best Buy alongside me when I was in college. I've met so many people in my life that have not been privy to the dynamically lateral career system you seem to be aware of.
I know from direct experience that some people do indeed fall off the ladder. To me it isn't a concept: it's the reality I've observed.