SixofNine,
Conspiracies, while not impossible, are nearly impossible. The more people who know, the more impossible they become. If some powerful world ruler with nefarious intent knows about aliens, how many other people have to know about it first, in order for said Dr. Evil to know? Lots. It hasn't happened.
You make an excellent point here--the more that know, the less likely it can be kept secret. As an example:
A persistent rumor has been that the US government has one or more crashed UFOs and/or aliens in their possession. Supposedly they're recovered and guarded by an elite group known as the "Blue Berets." This has been going on at some top-secret military installation for decades.
Many objections have been made to this scenario that tie in well with your point. Basically it comes down to how many people MUST have found out about it from being involved in the retrieval or study of the material, plus those seeing the reports, plus several Presidents, etc. Why no beyond-a-doubt eyewitness testimony, photos, alien artifacts, files, etc revealed? How about proof we've applied alien technology somehow (Stealth bomber)? How are these people kept silent? Are they all murdered? How to do that on such a scale that future participants don't learn of it in time to avoid the same fate? And are all Presidents kept in the dark?
Victor Marchetti co-authored "The CIA and The Cult of Intelligence. The Agency went to court to prevent its publication, but they failed, but they were able to win hundreds of deletions as a condition of publication. Yet nowhere does Marchetti or his co-author mention UFOs! Nor have other "tell all" books written by former spies for America, the UK, or other countries mentioned them either.
I taped that "alien autopsy" on TV hosted by Jonathan Frakes ("Commander Will Riker," Star Trek:TNG). Let's face it, if that was real, there was very poor camerawork for such an important project. The video whited out completely every so often. The admission that parts of it were digitized didn't help (remember how Forrest Gump was made to look like he'd met JFK?). And the film's revealer had a big grin on his face the whole time that makes me distrust him.
Impossible? No, but logical objections make believing in such an ongoing conspiracy extremely unlikely, doesn't it?