That gets into modes of engagement, albeit at a potentially more granular level. If we accept that canon is the conceptual framework that defines a particular work of imagination - one whose definitions are set, externalizing it from us in a way that stabilizes the particulars involved - then it's not hard to see how some people can come to a point where it's stabilized enough that they don't care about further canonical development; they've gotten to a point of sufficient definition, and from there on in feel comfortable further defining it as they like.
But that point is going to be different for everyone (and, for that matter, different for each particular franchise). Some people won't have that happen at all for a given body of work, and are always reveling in further development of canon. The thing there is that the cessation of further development, or the de-canonization of old work, therefore adversely impacts them, whereas those who've already reached their point of maximum value from developments won't care (or at least, are less likely to care).
Now this I could understand and sympathize with. If people felt that the canon was incomplete and was being abandoned, I fully understand that.
My issue with that interpretation is that... no one has really put forth that position. No one is really saying "but now we aren't going to get the closure we need for Greyhawk" or "but what about this hanging thread in the Forgotten Realms". What people have been saying is that the old lore is being trashed or destroyed wholesale.
If this is an issue of "but now this story won't be concluded satisfactorily" then I get it, but I also feel like that is a weird attitude to take with an RPG.
I'm not sure using a concrete example for a conceptual framework is the best way to go about analogizing things, at least with regard to this particular topic. I'd make an example of the king announcing that he's reached a deal whereby the entire island is now under the political control of a foreign power; a mainland empire, if you will. Your personal life might not change much - the foreign rulers might not ever visit your homestead, only showing up once or twice a year at a port on the other side of the island - and your taxes now go to the local king to be passed on up the political food chain, rather than being his alone. But it's not hard to understand why some people might find such a situation unpalatable, despite no practical difference in their everyday lives; the political (i.e. conceptual) framework that they're operating under has changed dramatically.
Yeah, analogies are always rough.
But, I was going forward with this idea of canon being the structure, the ground you stand on, the stabilizing force. And, even in your counter-analogy... it doesn't work. Because if you don't buy into the new canon and keep with the old... then your "taxes" aren't going anywhere different. You are still engaging in the old paradigm, just there is nothing new being built by the rulers.
To clear things up: the existence of non-canon things - when made by people with no (perceived) authority to determine what is and is not canon - isn't really an issue. No one cares that there's fanfiction about Middle-Earth, or Harry Potter, or My Little Pony, etc. so long as it doesn't come from Tolkien, or Rowling, or whoever at Hasbro gets to decide what's in MLP.
To point out though, Fanfiction contains its own canon. The author of a fanfiction work that creates a new reality for the material, contains a canon. It just isn't the official canon of the official property.
The issue is when the established canon is suddenly redefined.
"Canon" (in my understanding) is an acknowledgment of status; it necessarily requires an acceptance of externalized authority with regard to our imaged worlds who arbitrates what has that status and what does not. By adhering to that guideline, we're able to understand the work in question as having definition, which gives it a "realistic" quality (to use a very loaded word), since it now shares an important aspect with the real world: both are beyond our ability to simply alter them at will. That grounds the fantasy world in a way that I and many other people find pleasing.
That might seem odd, to consider that we're essentially giving up what seems like a large degree of personal agency with regards to something as personal as imagination, but it's important to remember that this surrender is entirely voluntary. As I noted previously
, nothing prevents someone from saying "this level of definition is sufficient for me" and ceasing to care about further canon; but likewise, that point will vary for everyone.
In a very real way, it's about how boundaries help define the spaces within them, hence why RPGs are essentially make-believe with rules; the rules limit what you can do, and so we find more fun playing D&D than playing Calvinball.
See, I think though that you are... overstating perhaps? Not sure the proper term to use here.
Yes, canon is an external authority deciding what is "true" in a fictional world. And that trueness is a boundary line. The issue is... the boundary lines are still there.
Especially in a case of DnD, what is "canon" is changed all the time for the group. For example, in my games Levistus, Lord of Stygia, is dead. One of my players had an epic adventure that led to her killing him and taking his throne. That is something I am continuing into my games. Levistus is canonically dead. That was important for that player, and I'm not reversing a great story.
But that isn't the "real canon" is it?
And this gets into the oddity about Canon in DnD. Everyone's adventures are canonical for them, but all alter the canon. If a group decided to kill the Harpells and burn their manor, then they might be really frustrated if the next time the DM runs them through Longsaddle, the Harpells and their manor are exactly like the canon says. It invalidates their choices to stick to the "real canon". So, you've moved those immovable boundary lines that defined the fiction. Maybe later you'll move them back, maybe not.
And so, since we already shift the rules. Since we already have people who say "the spell plague didn't happen in my realms" "The Sorcerer-King took an apprentice in my Athas" "The Paladins of Helm never waged a religious crusade against the Tabaxi in my Realms" ect ect ect, then whether or not the lore you are using is approved canon instead of old canon... seems like an arbitrary distinction. You can let the old canon define your limits still... it just isn't defining WoTC's limits