A thought about how decadent society leads to knowledge loss

In the past months I have been reading books and watching documentaries in youtube about the wisdom and feats of the ancient China, basically stating that a lot of the "modern" knowledge in engineering, astronomy, navigation, metalurgy, botanics or construction, among other disciplines, comes actually from China, and that it was discovered and mastered more than 2,000 years ago (maybe less for some of the artifacts and knowledge).

Even if the proof about it shows that we should review some of the official history of Humanity, shifting a lot of the discoveries towards the East... I have this uncomfortable feeling about how wisdom can be lost. The same can be said, maybe at lower scale, about the decadence in the West. How is it possible that the order, knowledge and capacity of the Romans, Egyptians or Greeks was so much lost after the fall of Rome?

It is discouraging how knowledge has always been under the control of a few, and how easily it was lost once the social and power structure that kept them is lost. Maybe not all, but a lot of it, and it is certainly a matter of decades to lose all that. Knowledge, with the writing idely available, should be the one thing that takes more effort to lose. You have to kill all those who know, destroy the results of that knowledge (buildings, artworks, artifacts) and also destroy the texts that explain how to do them. Considering its use (we are not talking about some filthy knowledge about minor things, but important knowledge that can always be helpful)... I find it hard to believe that people, normal people or simply technicians, would be willing to destroy, or simply not conceal, that knowledge from the destructive powers. First, you should have those destructive powers... and maybe there were.

In China, apparently, the political turmoil led to power to a certain cast that was interested in banning knowledge or keeping it as secret as possible. I am talking about the Mandarins (in the XV century). I would consider a similar process in the feudal lords in Europe... who would disdain the knowledge and the effort to nurture it (educating labour? what for?). I can also see the religious powers wary about the scientific knowledge. The example of the slow recession of the Catholic church from the matters that today are treated by science, but that some centuries ago were, together with anything else, dealt by the scholars of the Church. There is, in the end, a power struggle... a process by which the knowledge is let go, kept or transferred. And considering knowledge is power (the first ones to travel, the first advantageous weapon, etc, etc)... I can see that politics want to control sources of knowledge.

However... from today´s perspective, I would hope that knowledge would remain accessible despite a big catastrophe, a big war or a slow decadence of our society, worldwide. I would hope that the plans, the wisdom, the history, the knowledge, in a word, could be kept safe and transferable to future generations, even if there was a big cut. Unlike Alexandria´s library, it should not necessarily be a temple of open knowledge, for we should foresee that knowledge should be kept safe from the wrong hands. Imagine it was restricted to a certain tribe in a post apocalyptical future. We would be repeating the history all over again. Not such an easy decision, how to keep today´s knowledge and how to make sure it´s not lost for future generations... if we find ourselves in decadence.