Or, as I said, they see canon as having intrinsic value in and of itself. Canon makes the world "coherent". Changing canon makes the world incoherent, obviously. Thus, canon has value.
Changing canon
poorly makes the world incoherent. For example, sudden decisions that a popular storyline (or entire continuity) no longer happened, or retcons that require popular characters to behave out of character. And such "damage" to canon is usually, eventually, routed around (i.e. ignored by fans and creators alike) or itself retconned, restoring that sense of coherence for most fans. The majority of canon fans are perfectly fine with changes if they're either explained in-universe, or make sense with what came before.
1. Most properties are not managed over the long term and so all sorts of internal inconsistencies crop up - Star Wars is the poster child for this. Where are my Anthropomorphic rabbits?
Star Trek. Doctor Who has so many inconsistencies that any notion of a coherent world is laughable. Quick, what is the population of Waterdeep? How big is Khorvaire?
Most canon fans, when faced with inconsistencies, either a) ignore them as insignificant while still appreciating the rest of canon b) pick the answer they like best, since there's no one "true" answer or c) partake in the great fan pasttime of explaining the error away. For example, Doctor Who famously has multiple Atlantises... but it's just left unresolved, as one of those things. They didn't have to wipe out
The Underwater Menace to make
The Time Monster work, or vice versa; most fans would agree that both adventures "really" happened, the Atlantises are a continuity error. Such mistakes haven't kept generations of Whovians from enjoying the series canon, or required them to treat every story in a vacuum; they find a happy place somewhere in-between that may differ from fan to fan.
2. Most properties were not designed with the idea of being able to add stuff in later. So, you wind up with things getting fuller and fuller and, once you step back, more and more ridiculous.
Sure, but most long-running franchises adapt in various ways - retcons, distant sequels to get some breathing room for major changes, treating things in broad strokes after a certain distance has been achieved, or even in-universe reset buttons. And those franchises - assuming they remain valuable properties to someone - usually survive just fine, still retaining fans who regard their canon as having value, despite whatever changes were made.
3. Because we can only add canon, never remove, any mistakes made in the past become virtually impossible to correct. The current issues with inclusiveness in the hobby is a poster child here. People absolutely freaked over a pretty benign warning sticker being added to older products. And, if we're not allowed to remove canon, then all those sensibilities from the past become enshrined in the game and cannot be changed. Not a good look.
Mistakes in canon are fixed all the time. Out of character moments are explained (mind control! clone!), embarrassing portrayals are reimagined in later works (the superhero actually treated their minority sidekick with respect, what are you talking about?), and occasionally a work is just plain ignored while the rest of the canon chugs along with fans (what are The Goliath Chronicles again?). You can find examples of such things on basically any fan wiki. (I could also link you to TV Tropes, but that would be cruel.) 5E itself retconned elements of
Curse of Strahd in
Van Richten's Guide, yet both books are to be taken as canon under the current policy...
4. New things do come out. Look at how the Discovery was designed compared to the old TOS Enterprise. Of course it looks more futuristic. A number of the things that we totally take for granted now didn't even exist in the 1960's. Never minding the SFX abilities to bring them to the screen. So, yeah, the bridge of NX-01 Enterprise looks more advanced than the bridge of NCC-1701. Of course it does. But, it doesn't stop people from complaining about it.
Most fans of canon Trek (eventually) accepted Enterprise as canon, even if there's still griping about specific plot points. Discovery, too (though there's more griping, it's still on Memory Alpha). Most canon Trek fans accepted explanations, or suspended disbelief, and did so without rejecting all of Trek canon as incoherent and valueless. Others may ignore Enterprise or Discovery specifically, but haven't rejected the rest of Trek canon up to that point. None of that required them to decide that canon itself had no value. They just found a way to accept the largest segment as "true" that they could.
I expect most D&D fans who had treated 5E as part of the larger canon will react to this announcement in a similar way. They'll either embrace 5E as "true" and the old stuff as optional, or reject 5E as "false" and stick with the canon up to 3E. Or come up with some other combination that works for them, still without deciding that canon no longer matters to them. (Fans who never cared about canon will be as they always were; they're unaffected.)
I just think it's a shame that Wizards would encourage such a split, when they could have just said nothing, done whatever retcon they're planning anyway, and let canon fans ignore it (or rationalize it) while still treating the rest as part of the bigger story. But it's done now. (Unless they backtrack when the official blog post comes out...)
