A Rule Of Thumb

There is a rule of thumb in sf but it can be broken. The rule is "save the appearances." What do I mean by that phrase in this context?

A premise of much sf is technological and social change. The future will be unlike the past and present. New things will happen in the future and the future begins tomorrow. Tomorrow, a large interstellar spaceship might enter the Solar System, approach the Earth and hover above the UN building. But, if there are ETs already here, then they are concealed or disguised. Thus, the appearances are saved. There has to be a sufficient explanation of why history went as it did and why the present is as it is even if tomorrow everything will be different.

Thus:

the Cavorite sphere was lost;
the Time Traveller never returned;
the Martians will arrive tomorrow - even though, in this case, we read an account of their invasion after the event;
in various works by Poul Anderson, aliens, immortals and time travellers are concealed among us now.

It goes without saying that immortals or time travellers conceal themselves. We do not know anything about them, do we?

But how can the rule of thumb be broken? First, the story can be set in an alternative present - and Poul Anderson did this also. His goetic timeline will not diverge from ours tomorrow because it has already diverged earlier when Einstein and Planck cooperated on rheatics instead of independently originating relativity and quantum theory, respectively.

Secondly, much superhero fiction maintains the pretence that political and economic structures, even including the identities of current post holders in high office, would be able to remain unchanged despite the public presence of a host of superhuman beings whose mere existence should change everything utterly. Thus, their timeline is the same as ours except that superhuman beings appear in newspaper photographs instead of in comic strips.

Alan Moore showed twice that superhero fiction can only be set in a genuinely alternative timeline:

the US wins in Vietnam if it has a superhero;
the Warpsmiths teleport weapons of mass destruction into the Sun.

And, in the goetic timeline, World War II was fought against a Caliphate and with magic. Poul Anderson worked only in prose, not in a visual medium, but his ideas were at least as fantastic as anyone else's.