Compared to untenured but tenure-track academic faculty (most of who will later get tenure), tenured professors put in less effort, are less focused on doing big-win projects, and are less willing to change locations, research sub-fields, or classes taught. They publish less, and change their minds less. But they are also more pompous in their speech; they complain more in public about outcomes for they and their local groups, and are more willing to advocate radical changes in other parts of society. Though they are willing to act to express their outrage when they have positions of authority that aren’t threatened by such actions. (E.g., grades, hiring.)
This seems a plausible guide to how our civilization changes as it gets rich and old (due to falling fertility). We are now all “tenured”. Not only do the richer older people among us get this way, everyone does to some extent, as we are all so influenced by each other. We complain more loudly and frame the minor inconveniences of others as moral outrages that they must not tolerate. But we also less change jobs, locations, and our minds.
This is why I don’t believe that that our increasingly polarized and complainy society is about to actually fracture. We aren’t about to go wild going to war with each other. We talk bigger about how others shouldn’t tolerate the many many injustices we see, but we aren’t going to do much about it except via complainy mobs, or when safely empowered by positions of authority.
This (inflexible entrenched interests) is basically the thesis of Mancur Olson’s The Rise and Decline of Nations in the 1980s, and in a more generalized way was also the basic mechanism of civilizational decline in Carroll Quigley’s Evolution of Civilizations from the 1960s. I think you are spot on, other than that this mechanism doesn’t have to depend on a population decline. I think Quigley would argue that population decline is a symptom of a civilizational already decaying in other ways.
typo: compliany -> complainy