[Obenuearema]

knowledge mostly stayed in the upper classes, among mags, shamans and such. when they were gone, no one was left to pass it on. and they stayed isolated.

True. Knowledge is esoteric. That happened among the Mayans, for example, where only the elite knew how to read and write. After the Maya decline, people continued speaking Maya but nobody could read the glyphs, up to the 1980s. The same happened with the Inca quipus, that today nobody knows how to read.