I mean, that’s such a broad take, any system can change, doesn’t mean it’s inevitable

It also doesn't mean that they're all the same system. So, if capitalism is one of the many systems that can backslide into authoritarianism doesn't mean that authoritarianism is a part of capitalism, despite your claim to the contrary.

We have had systems of feudalism and monarchies that have stayed steady for hundreds of years. In pre history, people lived in communes for thousands of years.

Yes, in the modern world things change much more quickly. Technologies didn't change for thousands of years. That meant that the number of people a farmer or a plot of land could feed stayed constant for thousands of years. That meant the maximum size of a city was pretty constant. That dictated the kinds of governments that were stable.

It was technology that has made systems unstable, not capitalism.