Interesting observation - I think another way to describe this phenomenon is that every society has to convince it's members to accept their individual role as members of society in order for it to function (you might argue that is society's only purpose). We can't all be kings, senators, business tycoons, etc. -- someone has to be the janitor or the street sweeper or the factory worker, and society's basic purpose is to re-enforce the individual's choice in accepting their fate.

Long enough ago, this was as simple as raising certain classes and races of people as slaves and telling them what to do and enforcing it with violence. However it seems that no matter how much you try to control it, societies eventually achieve enough self-awareness of the general inequality of things to revolt and re-order society to make things more fair. The next system of control is then instituted, and is seen as a vast improvement, and thus accepted by those who rebelled against the previous, even though it may be simply a more complex version of the previous system (slave-owners/slaves,land-owners/indentured servants,royalty/peasants,ruling class/working class, etc.).

In this way, I believe it's similar to the quote from HHGTTG: "There is a theory which states that if ever anyone discovers exactly what the Universe is for and why it is here, it will instantly disappear and be replaced by something even more bizarre and inexplicable." I think that what happens is every time a society reaches the level of self-awareness required to realize what's happening to them and has the will to change it, a simply more complicated and harder to understand and harder to destroy system ends up replacing it.

I think in this way, you can describe the cyclical nature of societies coming and going, and applying the reasons behind this fundamental change, understand why your theory of war being beneficial fits right in: war simply leaves no room for questioning society's demands of the individual. The reason this works is that members of a militarized or war-time society are indoctrinated, and those who end up members of the military trained from day one to obey orders and follow authority. After leaving the military, if you do you happen to survive and get out of the war, working a factory job and having a few kids is damn near paradise compared to the hell you went through. It is a very effective way to keep individuals from questioning their place in society or the fairness of it's structure and thus why I think it has the society-prolonging effect you've observed. War shifts individual's thinking from long-term (career, legacy, mark on the world, children's future) to short term (survival, employment, simply procreating).

If you buy into any of what I am saying, you might also hypothesize that the U.S came close to becoming self-aware of and motivated to change it's societal inequalities around 2008 with the reaction to the Government bail-outs and the housing market crash. I think the only reason that energy hasn't really gone anywhere yet is twofold: 1) With the economy in the shape it's in, it makes those who are employed more happy with their current employment situation than usual, and more unlikely to try to change things in general, since they are "lucky to even have a job in this economy", and aren't really feeling the inequality in their day to day lives. And it's not bad enough to where mass unemployment is giving people enough free time to organize and actually try to restructure the way our government works. 2) The act of electing public officials to office helps stave off the idea that "we need to restructure society" as it lets people restructure a symbol of society to them. Elections act as a relief valve for political pressure, since it allows people to 1) have input and make changes and 2) it buys time for the newly elected official to "get settled" in their office before people become dissatisfied again or economic/societal conditions improve, whichever comes first.

Sources: Just my theory on the matter, please point out where I am wrong (I'm sure I am in more than one way)