Consider the following reasoning:
Humans have different values and ideologies.
In a democracy, there is always a group that is willing to violently impose their values and ideology over others, arguing that democracy is rigged or flawed and thus not valid (for instance, Marxists in capitalism, capitalists in socialism, anarchists in all of these)
To "protect" democracy from these violent views (including "foreign influence"), society imposes restrictions to democracy (for instance, outlawing Marxist political parties, or having a one-party system (e.g. China)
From these three premises, we can deduce that a "perfect free and open democracy" is not possible (or at least not a sustainable political equilibrium).
This is, to my view, a very pessimistic result because it justifies a less democratic regime, without specifying the direction of the bias (i.e. pro-capitalism, pro-socialism, etc). In other words, anything goes.
Is there a flaw in this line of reasoning? Is there a political theory that describes this "impossibility of a true democracy"?