"UFOs are spaceships piloted by beings from outside our planet, and our officials have been making regular contact with them for decades."
It's highly unlikely you will ever hear a sitting, high-level official make this declaration. Why? Because within this statement are implications which would likely destroy the fundamental foundation of our society. Listening between the lines, we actually hear these other upsetting statements.
"Our security has been irrevocably compromised."
Suddenly, we realize that we are the aborigines trying to fight gun-bearing Europeans with spears and arrows. Aliens will have infinitely superior killing technology, and this is just a fact. Whether it's planet-blasting lasers, or handheld weapons that instantly immobilize/kill/shrink any human within range, we can't compete with the technology of a group of individuals with the ability to fly here from another planet.
In security negotiations, we will never be on-par with the aliens. Any negotiation will be a de facto discussion of our terms-of-surrender. How can you lead and defend your nation when your citizens know that their survival really just boils down to the whims of the aliens buzzing around overhead?
"There's a new status quo, and you're on the bottom."
Around the neck of a recently-excavated pharaoh, they found a pendant made of steel. Steel was high-tech and rare, and worth more than gold. Despite all his wealth and power, Caesar never had access to a television. But had he known about them, you can make damn sure he'd have acquired one. So what about all the sweet new toys that aliens will have? A Porsche just doesn't look as shiny next to an interstellar spaceship. A beach house in Hawaii? How about a beach house on Vega. Telling six billion people there's really cool stuff out there they can't have is a sure-fire way to cause massive unrest.
While some super-rich humans will no doubt use their power
and influence to find a way to get some alien toys, the vast majority of us will not. And we won't be pleased about it.
"There's a way to live forever…but not for you."
Yeah, they'll have the technology to accomplish this. It'd be naïve to think they wouldn't. But just like that beach house on Vega, your US dollars won't buy you immortality. Nothing scares us more than death, and nothing pisses us off more than being told we can't have something. Put these two factors together, and you'll see the realization of a line from the movie 300 when we 'put their immortality to the test.'
"You'd better not get out there and start blasting UFOs with your pistols."
People would do this almost immediately. You think racism is a bad thing? How about the xenophobia bred from such an obvious imbalance of power? And if the aliens have the ability to look like us, you can bet we'll be having some issues of trust with every person that crosses our paths.
"We, your government, are a bunch of liars and you can never trust us again."
This one should be obvious. If they ever admit that they've been keeping this info from the masses, you can be sure that their credibility as leaders will be shot. They would lose their jobs. And we won't trust anybody else to take their place as leaders. And the bottom line is that this factor has to outweigh all the others, because at heart we are all self-serving enough not to do something that will jeopardize our livelihoods.