Not sure I agree with this.
1) As you admit, conspiracies occur. We know this because some conspiracies in the past have failed and been revealed. Some have succeeded and been revealed (Iran Contra, for example, was successful, in that some of the goals the conspirators sought to achieve were achieved and everyone involved basically got away with it). Note that 'revelation' is not the only possible failure mode for a conspiracy.
2) I observe that to a large extent society is heavily influenced by a large number of quite powerful organisations composed of individuals of various levels of public profile and accountability. This observation is consistent both with a good-faith interpretation of what the saner wing of the para-political community claim and with any number of entirely rational theories about how the world works.
3) Pace Popper, of course some things 'just happen,' but sometimes things happen because small groups of people secretly and illegally and unethically cause them to happen.
4) One doesn't have to believe the 'conspiracy theory of society' to have entirely legitimate concerns about the role and behaviour of the secret state.
Shorter: to claim that a conspiracy theory is automatically nonsense by virtue of someone describing it as a conspiracy theory is unwise.
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