Canon is ultimately a product of the mind. It is suspension of disbelief. When canon is violated, it risks shattering suspension of disbelief. This is why we have crazy threads here trying to rationalize things that were obvious plot-holes, goofs, or poetic license.
Here is one active "rationalization" thread.
There is no in-universe explanation for why the refit Enterprise looks so different from TOS. They tried to hold onto the TOS-isms on the Phase II version but they decided they had to go pretty much blue-sky to make it really pop, and it was ultimately the right decision. However, it creates mental "friction" in sustaining suspension of disbelief because of the way the screenplay still describes it as merely a refit rather than an all-new ship. What really should have happened is to have treated it as a whole new class similar to the -B, -C, and -D, but since the screenplay was such a moving target even up through shooting they simply could not reconcile it.
Trek fans hold trivia about the behind-the-scenes aspects in a separate part of their brain from the suspension-of-disbelief part. The S.O.D. part is the escapism, the Oz, the Neverland.
When we're in the zone of S.O.D., these continuity errors or deliberate violations for the sake of modernized production values create friction. It's like imagine if you went back to look at old photographs and they kept shifting to be more modern rather than being a true time-capsule. Our memory of TOS is inviolate. That's one reason why some people have a problem with the FX on TOS Remastered. I don't, but some do, because that is their mental pocket-universe, warts and all.
So when the new stuff comes in, it feels like a mental violation in a way. It's some snot-nosed punk saying "this is how it SHOULD have looked if they had more money and technology and better taste". You know,
fashion. This is a big reason for the blowback against the Kelvin-verse, for those who couldn't take the excuse of the alternate-universe aspect.
I could go down the line.
Now, in these sorts of threads there are armies of people who say "you can't expect things to stay static. It just wouldn't sell". That may be true, but at the same time, you have to at least pay a modicum of respect to how much people value their imaginary worlds that they carry around in their heads. Within that dreamscape they really don't give a damn that the walls were cardboard and the switches were cast from icecube trays. That's what they were introduced to when they were little kids and they don't want anyone retroactively telling them otherwise. No bloody -A, -B, -C or -D as Scotty would say.
So ultimately all canon is "head" canon.