While this seems to be generally true of almost any artist's work (see the David Bowie dialogue in Trainspotting for elucidation) I have to strongly disagree when it comes to two SF series that I have read in their entirety. (Warning!! potential spoilers ahead!)
Isaac Asimov's Foundation series is exhibit A. While it is not cited above, I believe it disproves the law (and as Dr. Asimov himself said, exceptions need to be taken very, very seriously). The first three books he wrote were published in the '50s and were probably the best sci-fi seen up to that time. He picked up the series again in the '80s, with Foundation's Edge, Foundation and Earth and Prelude to Foundation (last written but first in chronology - although it should still be read last, a similar situation to The Magician's Nephew). These later works are longer and, to my uneducated belief, incorporate more effectively a description of how a society's unspoken hopes and dreams (read: myths) effect its history. And, it seems to me, that's what the Foundation series was all about to begin with.
Exhibit B is Frank Herbert's Dune series, included above as proof of the law and an inclusion I have to take issue with. Dune is a very emotionally satisfying read: uprooted-teenage-boy-with-no-friends-his-own-age-combats-evil-pedophile-and-warmongering-emperor-and-beats-them-to-become-emperor-(and messiah)-himself. But Herbert was not out to satisfy anyone's emotions or to portray his world in terms of black and white. The whole reason he wrote Dune in the first place(according to his interviews, and common sense) was to build up a messiah figure (Paul) and then tear him down. He does this beautifully in Dune Messiah and Children of Dune, the latter being easily my favorite in the series. In God Emperor of Dune he goes a step further. Having already exposed the fallacies of the Atreides, he sets up Leto II as "The Tyrant," who rules for 3500 years and does some truly horrible things. But he says he does this only for the survival of humanity, and this may just be true (of course it was Paul who jeapordized it in the first place) Before Leto dies we catch some fleeting glimpses of the tragedy that he endured in his life, and are allowed to feel compassion for him. So just who are the good guys here? In Dune the answer was simple. In the rest of the series it is not (at least books 2-4 - the Bene Gesserit seem to wear the white uniform in the last two, but you still can't be positive). You have to think more as the story progresses, and that's why they're better books.
So how do I deal with these exceptions, while still recognizing that Wharfinger's law holds true most of the time? I don't know for sure, but I suspect that it may have something to do with the motives and motivation of an author. If you accept that anyone does they're best work when they're hungry, you have to ask yourself, "hungry for what?" Our culture teaches us that large amounts of money are the only thing worth hard work, but it may be that some artists still produce for love of their art alone.
....Hey, wait!! You can't take away my membership card in the Young Cynics of the World club just for that...I...I said some...and maybe...Where are you going...Come back here with that...It's mine, I earned it, damnit!!!