1 Young Adam: Right. We've effectively removed our supposed grandfathers from the timestream.
2 Young Adam: We must be our own grandfathers now. Myth confirmed!
3 Young Jamie: There are bigger issues here. Martians do exist and the government has been covering up the existence of flying saucers since the 1940s!
4 Young Adam: Yeah, but everyone already knew that.

If the US Government actually admitted to having covered up the existence of aliens since the 1940s, it really wouldn't come as that much of a surprise to a lot of people.

Even the people who it did surprise wouldn't be very surprised. We've been nicely conditioned by the conspiracy mill to have a sort of cultural semi-belief that it's happened this way all along.

If you were a government looking to cover up some Earth-shattering news like this until a time when it could safely be revealed to the public without causing a panic, you'd create a seemingly discredited rumour mill, seed the popular fiction media with science fiction concepts, and prepare the public consciousness generally for the idea of aliens until we no longer think of them as anything all that unusual. In other words, exactly what has been done for the past 60 years...