So it is with my opinion of conspiracies. Everybody already knows that JFK was killed by [insert villain here], and that UFOs come from [insert planet here], and that [they] are putting fluoride in our water, and that [some international cabal] is controlling the world, etc, etc.
The problem is that I don't really know the truth about any of these things. I suppose the truth of each subject in which conspiracies are proposed is probably pretty boring. There probably was nobody but Oswald who schemed to kill JFK. There probably aren't any aliens from anywhere and there are very few international groups which can effectively control the world, although Lord knows they try.
Yet, even choosing the most boring possibility is no guarantee of throwing cold water on a conspiracy theory. That's the beauty of them -- they are always proposed in such a way that no one can prove anything about the conspiracy -- that is unless the actual villains come out and admit their scheme, complete with proof beyond a shadow of doubt.
No one can disprove the JFK conspiracies, because it is always possible that Oswald had accomplices who got away with it. It may not be proven that any extra bullets were needed, it is possible that just the 3 shots from Oswald's gun was enough. The only other issues would be the motive. It seems nebulous to me as to why Oswald would shoot JFK. Just for the hell of it? For money? As part of the CIA? Who knows?
Aliens are a crazy bunch. They fly around the star systems, looking for poor, unsuspecting sentient beings on various planets -- subjecting them to invasive probes in the most private of areas on their bodies and casting them aside like so many used prostitutes. OK, I suppose that could be chalked up to scholarly pursuits. Kind of creepy really, yet there no proven kidnappings by aliens -- just a kind of speculation about what might have happened to many missing persons.
I won't bore the reader (myself?) with an endless stream of examples. But perhaps I could simply create a conspiracy of my own. I need another person, of course, otherwise it is merely a secret. There must be more than one person in a conspiracy. So I could select my wife as a (possibly) willing co-conspirator. Our subject could be that I know a woman that is selling a house for less than its market value. She will demand a deposit, I will perform a "credit check". Then she disappears with the deposit check, but I actually perform the credit check and determine that his credit is [whatever it is]. I have the person's trust, but my wife has the deposit check.
So, I promise to retrieve the check from this "rogue" woman, but only the portion greater than the credit check (some arbitrary fee which is greater than the fee charged by an Internet credit service.) I return the partial amount of money and pocket the difference. The person is glad to get away with their shirt and the knowledge that their "credit is good."
I do not have the actual courage to do some illegal thing like this -- the payoff would be way too small to risk some kind of long prison sentence for fraud and conspiracy. And certainly I would not want to also risk my wife's freedom too. So the motivation to actually insufficient.
Instead, if the motivation was to hide alien technology from stupid humans that would use it in the least responsible way immediately (as is usually the case in stories about Genies granting wishes...), then a conspiracy of secrecy might actually be in order -- so long as the group truly was responsible and did not simply hog the technology for themselves. That is probably the more likely outcome of such a conspiracy. Yet that very fact is what might make such conspiracy hold together, even over a large, multigenerational group of people.