^It's not about vocabulary. There is so much unnecessary arguing and vitriol about canon based on misunderstandings of how it works, and it's that bitterness and negativity and confusion that are the real problem. I'd have no problem with a shift in definitions if it had a constructive impact or a neutral one, but that's not what I see here. I just lament seeing so many people being angry or frustrated when they don't have to be. It's not about what words they use, it's about the myths they embrace regarding the nature and process of canon, myths that cause all that needless frustration.
But what about the people that use canon (by the fandom-jargon definition, which I'll be intending throughout this post) without any anger or frustration? That use canon to mean continuity and don't care when something is out of canon? Most of the fractuous behavior towards canon (in the fandom sense) that you're describing is from the old fandoms when the word was first transitioning over; Star Trek, Star Wars, DC/Marvel, the pre-Internet fandoms largely. It's continued to cause fractuousness in those largely through inertia more than anything. But look at the Avengers fandom, the Buffyverse fandom, Supernatural, Sherlock, Hannibal, the list goes on. Most all modern-origin fandoms talk about canon in the same way as the old guard fandoms, but I've rarely seen anyone worry about something being canon or not the way people do for Trek. The most recent fandoms use canon to mean continuity, yes, but they don't then also embrace canon as the only thing that matters or ignore the worthwhile aspects of non-canon things or fret about what's canon and what isn't. The only time they ever raise the question is when they want to know if a given thing happened in the context of a new story or not.
The anger and frustration you're worried about? It might still happen in some
fandoms, but an increasingly vanishing minority. And it's likely in part because of the definition shift, though likely a very small part. I mean, canon=continuity doesn't even really entirely describe how it's used anymore, what with discussion of headcanon and all, people considering tie-in works or even fanworks as on equal footing to them in their own conception of a given fictional universe and all. Describing what "really" happened in a given person's mental construction of the universe. For me, for example, nearly the entire modern novelverse is in canon, while for someone else, it isn't. In the most recently-developed fandoms, canon is far more akin to Grant Morrison's original conception of Hypertime, how each story can pick and choose whatever subset they want amongst all the previous stories for what happened and what didn't, even in a shared universe. Something not contradicting previous continuity, not writing out something that happened before, but rather merely providing a different collection of past events to build from.