Doctor Who's depiction of time travel, to put it mildly, is a mess. It makes so little sense, that its hard to have any real idea of what did and did not happen in the final timeline. Do the Daleks still invade and conquer Earth in the 22nd century or was that event erased during the purge of the Daleks from history during the Time War? How much does "modern" humanity know about the existence of extraterrestrial intelligence. The Cybermen very publicly invaded Earth as far back as the 1960s (or 70s depending on when those events were actually supposed to have occurred). Even excluding the stuff from the old shows, even the modes incarnation of the series seems to be all over the place when it comes to aliens on Earth. UNIT has been in existence since the 60s and Torchwood had been around since the 19th century (yet both manage to be both secret and oddly public at the same time). From "Aliens of London" to the various Christmas incidents Aliens have been shown publicly several times (and been on the news). Planets have appeared and the Earth has been moved. Yet strangely both Torchwood and Doctor Who keep showing us a modern and near future Earth where people are SHOCKED at the existence of Aliens. Torchwood: Children of Earth and Miracle day almost seem to have taken place on separate planets given that the CIA was apparently unaware of Torchwood or aliens. That would seem odd given the existence of UNIT, the fact that the President was ASSAINATED by an alien on the Victory and that the US government and public were aware of what happened during Children of Earth.
It is a total mess but the US President being assassinated by the Master was ret-conned when the time paradox ended. It never happened. I believe in the RTD Era, "Aliens of London" was supposed to be the first time that humanity learned about aliens. And then "Army of Ghosts" was another major public outing. In the Moffat Era... who knows what the hell is going on. He rebooted the entire universe in "The Big Bang".
No, actually, that still happened. Right after the reset, we hear someone over a radio demanding an explanation saying "we just saw the President assassinated." Anyway, it has always been a running joke in the show that humanity just conveniently forgets about aliens. The Seventh Doctor comments on this in Remembrance of the Daleks. When Jackson Lake commented that everyone will be talking about a giant robot stomping through London, the Tenth Doctor just says "maybe not." In Torchwood's pilot episode when Gwen acts surprised about aliens existing Jack gets frustrated wondering why everyone tries to deny the facts that stare them in the face. Basically, modern-day humanity in Doctor Who exists in a constant state of denial and refuses to accept the existence of alien life.
Good ol' 11 said that the cracks in the universe were responsible for erasing the "Giant Cyberman over Victorian London" and "Planets in the Sky", so that means that at least the next doctor and stolen earth were erased from the memory of humanity, not sure about the rest though.
It's Doctor Who - it's more of a fairytale than a hard science fiction show - people remember what the plot needs them to remember.
Well the show has pointed out on several occasoins that time can bre re-written. So it is entirely possible some future event in the Doctor's timeline could cause for example humanity to lose knowledge of aliens. After all we could be seeing an effect before a cause.
Actually the death of President-elect Winters was the last event that actually did occur. The Valiant received a message saying that the last thing that they saw was the President being killed. Plus Winters had to be killed since Obama is President a year later.
The constant state of denial is contradicted by events on the show itself. For instance in Voyage of the Dammed, it was mentioned that people had cleared out of London for Christmas precisely because aliens seem to threaten the city every year during the holiday. They were actually pleased when the day went off without an incident. Yet then everything seems to be reset afterwards.
An alien shipe crashes into Big Ben and not long afterward it's a claimed to be a hoax, the Loch Ness Monster rises out of the Thems and it's well forgotten as was the Yeti in the underground. The human ability for self deception is matched only by it's ingenuity while trying to destroy itself to quote the seventh Doctor.
I find it best not to worry about it. I mean Davros' plan to snuff out existance in the 21st Century doesn't worry about future history.
Come on have you learned nothing from watching this show? “People assume that time is a strict progression of cause to effect, but *actually* from a non-linear, non-subjective viewpoint - it's more like a big ball of wibbly wobbly... time-y wimey... stuff.” Also: -The Daleks will come back -The Cybermen will come back -Everything will come back -There is no escaping the madness On a serious note, I do believe that humans are aware of the existence of aliens because of several (beyond several actually) incidences that have happened, for example you mentioned the cybermen. During the Donna period, London cleared out on Christmas because they were expecting something bad to happen. However, like someone else here mentioned, they seemed to forget after. It's best not to try to make sense out of it maybe.
I'd forgotten that London evacuated in Voyage of the Damned. It's pretty lucky that they did - in the alternate reality shown in Turn Left I thought alot of people had died.
The beauty of Doctor Who is that it can get away with bringing aliens to Earth a million times, and then still have them experience their first ever encounter like it did in The Eleventh Hour.
Maybe the Tardis has visited Earth so many times that somehow the functioning of the ship's perception filter has left a permanent, but diluted residue on the population. To quote Ten: "[It] just shifts your perception a tiny little bit. Doesn't make things invisible, just un-noticed." After an alien invasion happens (and IS noticed and is subsequently repelled), the population gradually begins to regard the past event as 'Somebody Else's Problem' and forgets about it. (I am just kidding....) (but maybe not. Hmmmmm..... )
What people fail to understand is the show only follows the Doctors timeline and NOTHING else. History and events of Earth and the Universe are always changing, it is only the Doctors history we follow.
The Sarah Jane Adventures was always weirdly inconsistent about this. In one episode, Clyde's dad would mention seeing Daleks in Germany (during "The Stolen Earth"), while in another, people who believe in aliens would be depicted as credulous kooks(!).
Maybe it's some brilliant meta-concept every time that they get the audience to "forget" all the alien invasions. I suppose a less kind soul could make some comment about it reflecting the lack of consequence in the stories.
I think the cracks in the universe were intended to reboot people's knowledge of aliens. It's been somewhat inconsistent since then. Best to assume people are unaware of aliens unless the plot requires them to be aware of it and then you should assume they'll mostly forget right after.
It's the problem with trying to keep it real in an ongoing story. Fine to start with but you play out one story it's hard to pretend they are still in your universe. 007 would be even harder to explain (so they don't bother!) Each Bond is essentially a reboot, yet with actors carrying over from the previous incarnation in the same role. If you look at Winters in The Sound Of Drums, even as a straight Bush Jr substitute, that changes the course of US politics for the Whoniverse. Yet it then seems back on course with Obama, by The End Of Time. But potentially a future Doctor Who story could visit the White House in 2007, with George Bush Jr. as a historical figure. UK Policitics is even more muddled!