If that does model best mankind and society, maybe it is not that much cynical and pessimistic but realistic.
That said, it is only a model, a tool to better understand ourselves.
Therein lies the problem, though. Models when used by people are not only models. They shape and influence reality in return in a feedback loop.
Even if a cynical model "does model best mankind and society" as it stands, there is the possibility that using a less realistic one could help improve mankind and society.
For example, there are arguments that the renaissance was a result of the social and economic conditions created by the black death, not the intention of any human actor. The post-WW2 social wellness a result precisely of the war, and so on.
Not everybody agrees on what "improve mankind and society" should mean, so you're back to either submitting to a common concept of objective truth, where the "bad" but realistic model wins, or political arm wrestling for your ideas to win against the other's (on the basis of faith if you're in the middle ages, or whim if you're a postmodernist).
This was not about ideas -- it's was about viewpoint, which has enormous impact in shaping the world.
>For example, there are arguments that the renaissance was a result of the social and economic conditions created by the black death, not the intention of any human actor.
Well, there were other periods similar to the renaissance throughout history, periods not affected by the black death. E.g. the rise of ancient Greece city states (from the so-called "middle ages" of pre-historic antiquity), or the rise of the islamic culture.
And even if the black death was a major factor of change, the way the change took shape is all ideas and viewpoints.
In fact a common argument is that the renaissance was indeed a response to the conditions created by the black death, but the mechanism of change was a change in viewpoints ("let's celebrate life", etc).
It had an interesting cultural remnant so embedded that nobody notices it - desensitization to skeletons. Nobody reacts to skeletons in an elementary school classroom or doctors office in the west.
Yet Chinese variants of video games widely censor their apperances despite other brutality. One in a moba notably changes a spell icons from skeletons to a tortured bald man - more graphic to us but not them. China does have their own hangups but it highlights how weird it is fundamentally that we are okay with dead bodies reduced to their very core and reassembled. Keep preserved other internal organs around small children and people would ask what is wrong with you. The unacceptable ethics in sourcing were the main driver for the switch to plastic if I recall correctly. Cheaper now but they started when the fine details were more expensive and at risk of being less accurate.
I used the first to refer to general abstract theories (marxism etc) and the second to refer to more concrete ways of viewing the world that people share / adopt etc.
In the 60s for example there were several abstract theories about this and that, but also a shared viewpoint about the need of change, revolution, etc
As you say, for the renaissance, the change in viewpoints were caused by the conditions of the time. The power of social change ultimately lies in big changes in the physical world, not in civilised humans and their thinking and arguing.
As I mentioned, that last idea seems to me to be just a covert way to pretend we have any control, when we are actually rather effete and incapable of shaping the world into what we'd like.
You mentioned the western revolutionary sentiment of the sixties in another answer. If you believe Strauss and Howe (http://www.fourthturning.com/), they didn't have a choice. That generation believed whatever humans believe when they are put into the conditions they were in, repeating a pattern with little self-consciousness.
edit: the power of change -> the power of social change
I grew up in a poverty-heavy community, where people often felt hopeless, and believed that everything was aligned against them (a pessimistic viewpoint). I then studied Philosophy and learned to try and be as "correct" as possible, and thought it was always the best method to try and be correct about things (a realistic viewpoint). After a while, however, I realized that some of the most successful and happy people I knew, and (I believe) in the world, are incredibly naive, and that this naivety and optimistic viewpoint played a large part in their success.
Even if determinism/fatalism are true concepts and/or nothing matters in the end, you'll have the best life if you disregard those concepts and assume that anything is possible, and everything matters, and it's all up to you. Positive viewpoints, even if incorrect, can be empowering and create this positive feedback loop.