: ---SRSBZNS---
: The main issue I have with the "alternate reality" explanation is
: that in some cases, the fiction has had a huge cultural impact.
: Particularly in near-future speculative fiction, the world they talk about
: might not exist if the genre, or even the specific story, hadn't come to
: be. An example would be modern-day superhero movies about characters whose
: previous adventures have shaped generations - but those comics and older
: movies don't exist in the movie, so the world really shouldn't look the same.

eh...
Comic books and superheroes have had a big impact, but I don't think the world would look, in the time that you usually get in a movie, noticeably different. There are very few works, actually, that I think would produce that kind of change, at least directly (so not counting X inspired Y to do Z and that caused A to build B or start C). That's more the overview or the big picture though, you might mean how people act and react. There are more works that have made a real impact in that way, but I think they're still pretty few and far between.