Part optimism, part human nature, I suppose. Pendulum theory would probably work better as a double or multi pendulum theory, since there are so many factors that bring chaos to the reaction that a simple pendulum can't adequately express how the reaction will play out. It will change course, but the how far and and at what velocity is difficult to say.

I also wouldn't look at it as a left/right thing. It's more of perceived problem/proposed solution thing, with a strong emotional component. The greater the perception of the problem, the more open people become to varied solutions, manifesting in some form of populism or another. Harnessing that brewing populism towards your ideology's proposed solutions is what is probably most important in determining how things play out, but it's still going to be chaotic.

Historically, I think you get it right in the general observation of patterns of extremism and reaction. Trying to look at minutiae of historical events won't tell you much about a completely different context.

As an aside, I also don't look at the Overton Window being fixed in size. Sometimes, what's called movement is really expansion or contraction.