Thinking about it a bit more. Now I am reminded of two other series that I enjoy: Sakura Taisen and Sailor Moon.
The Sakura Taisen canon is really... interesting. There's the original games, of course, and there are also at least four OVAs (Gouka Kenran and the like), spin-off games, an anime, a movie, a manga series, several live-action musicals,
one TWO eight-disc compilations of music from the series (something like 17 hours' worth of music, because Kouhei Tanaka is a god), and hell, even
a restaurant. Each and every one of them is canon, too. And they all contradict each other in many respects (the anime compared to the games comes to mind), so you actually have to identify
which canon you're using in fanworks or references.
Sailor Moon: here, the manga is the original-- and even then, it's the
Sailor V manga that's the original, and the Sailor Moon manga was itself just a spinoff. Then there's the anime, which diverges pretty wildly in many respects (lol Starlights). Then we have the SeraMyu musicals, which are completely awesome and have their own separate canon and storylines. Pretty Guardian Sailor Moon, the live-action Sailor Moon, also has its own unique story. Then we have the SNES Sailor Moon RPG, and countless music CDs, OVAs, drama CDs, supplementary materials that fit in mostly with the anime and not the original manga. Like Sakura Taisen, Sailor Moon fanworks have to identify which canon they're using (example: "I'm using the anime storyline, with some elements of the manga, and Sailor Darkury from PGSM.")
I come from both of these two fandoms. Both have internally contradicting, separate sources of canon. Does anyone care? Hell no. Touhou, however, has a bare-bones canon (the games), with a few sources of supplementary material (BAiJR, Grimoire of Alice, etc), and that's it. So whenever ZUN introduces a plot element that may contradict what he said earlier (PC-98's relationship to Windows comes to mind), canonfags go batshit insane. Having only one ultimate source of canon in ZUN, canonfags have made the mistake of thinking he is
sober infallible, and actually cares about making his stories internally consistent. (To his credit, ZUN
has started to do that, ever since the events of MoF.) If Touhou were anything like Sailor Moon or Sakura Taisen, its fans would learn to deal with it whenever a contradicting plot element is introduced, and we'd have less canonfaggotry and fanonfaggotry on the whole.
In short, both of their fanbases are sane, unlike Touhou's. When they see inconsistent canon, they :dealwithit:.
On the other hand-- the dangers of
not having a fanon are way more pronounced. The Rurouni Kenshin fandom seems to have failed
specifically because of a well-detailed anime and manga (and a few OVAs, but
Seisouhen does not exist), leaving very little room for reinterpretations. Fanfiction is literally the only place where Rurouni Kenshin fandom now lives-- and
some scattered doujin communities, but they can be counted on fingers and toes, at best.
Or what Sabino said.
... wait why am I doing this instead of my biology homework ffffffffffffffs