Large groups of people absolutely can actively or passively conspire and all remain quiet about it if their incentives strongly align. And if one of them breaks rank, then we can just pay off the news media to ignore them or paint them as a quack.
It may have utilized up to 5000 covert agents, and was only detected perhaps due to an observant librarian.
It's worth it to be wary whenever someone accuses another of being a conspiracy theorist. Admittedly, some might be the actual disconnected-from-reality conspiracy theorist but one should always use a discerning eye when the claim is thrown around.
There are similar principles in military science about limited sizes of surprise attacks because logistically at some point it becomes completely obvious to anybody with working eyes or ears if say, the PRC relocated the entirety of their army off the shore of Hong Kong. Let alone the cumulative probabilities of human failures including in blackmail and bribery or how quickly the finances would add up in paying off people noticing you paying them off, and then people who notice the abnormal flow of currency.
There is no guarantee in these principles in the same way there is no guarantee that WWI wouldn't happen due to the interconnectedness of the European countries and how much they would have to lose. They were still right about the latter being an unmitigated disaster, as the unprestigious "backwater hicks who barely count as civilized", the US and the USSR became two world leading superpowers instead of the other leading empires. But that same "Unlikely because would be incredibly stupid and lead to losing a lot." level of guarantee applies to nuclear deterrence.
Usually this argument is super solid against the kinds of "long running super conspiracies" that people argue for. E.g: - The earth is flat and the fact it is a sphere is a lie told by generation of scientists and elites. 1000s (or more?) of people * hundreds of years. Ya right. - The moon landing was faked and everyone at NASA at the times knew it. 60 years * hundreds or thousands of people * massive resource spend and physical footprint: ya right.
It's also very easy to overestimate the likelihood of discovery, since it turns out all of these factors are correlated.
Remember, the conspiracies you find out about are, by definition, the ones that didn't stay secret. Now consider how many secrets you keep, and how many secrets of comparable magnitude you know about arbitrary other people.
However much of the annoying noise from conspiracy theorists involve people who all know they are doing illegal stuff and choose to do it anyway and keep quiet about it and they make it out to be some massive, evil, covert conspiracy party.