I really like your analysis. I think the fundamental reason why is democracy so vulnerable is that in practice, there is a lot of socioeconomic inequality in societies, and that really erodes the core democratic concept that everyone should participate equally in decision-making.

So in very unequal societies with a semblance of democracy, like in South America (and it's slowly coming to USA as well), you get these right and left populist oscillations, as different classes (upper and lower) seek to gain all political power. I don't think anybody has figured out an effective way yet how to dampen these oscillations in a society, and equalize it, so it would return to the ideal democratic mode.