All fiction is equally real.
That is to say, all fiction is equally unreal. Fiction, by its very definition, is make-believe. Made up. Imagined.
From the biggest summer blockbuster movie, to the most obscure novel buried at the bottom of a bargain bin, to the crappiest Fan Fic, to the smallest passing idea of a story in a random person's mind, all fiction is equally real.
Fiction may vary in a lot of ways, specifically in genre, quality, presentation, and cultural significance. However, the ideas underneath it all are equal in gravity. They are all exactly 0% real.
Fans of a show can take comfort in the fact that, while something happened on the show they don't like, they could invent an alternate continuity inside of their mind, and it would be equally as real. After all, it's just television.
This principle could be said to be the source of all Fan Fiction, or, to an extent, the source of all fiction; the idea to aspire to significance in writing is what drives people to use tropes to build their own work.
The Fiction Identity Postulate can be proved by illustrating that, when it comes to story concepts, all ideas have the potential to be good. Sometimes all it takes is a little tweaking, other times it may take serious revamping and changing of the quality, genre, presentation, audience targeting - in short, all the aspects of the work that exist entirely in our reality - but beneath every pile of crap is something pretty if you are willing to dig through it.
Of course, this can be completely inverted (or just explained in a different way) by an infinite multiverse: everything that has the tiniest possibility of existing must exist; this makes all fiction 100% real. We call this explanation "The World as Myth".
See also All Myths Are True. Compare Mutually Fictional, Recursive Canon. Compare and contrast Sturgeon's Law, which is more an analysis of the results than a critique of the foundations. The MST3K Mantra is an invocation of this principle to dispel the frustration created by Fridge Logic.