Asking an honest question as someone new to the FR and canon as it relates to DnD. What is the significance as to whether or not BG3 (or anything else for that matter) is considered canon? Obviously players are free to ignore or embrace whatever works for them. I'm assuming its only an issue for publishers.
The significance is that it enables future stories to include parts of the canon stories.
For example, if BG3 is canon and it's Steel Watch factories are canon, then future story writers can include Steel Watch factories. Which, if someone really doesn't want that in their FR, means it will detract from that particular story.
Whether this future story is a module (Thus more easily avoidable with a likeminded DM who just doesn't include them), or a video game, it can impact someone's overall enjoyment of the content.
With of course, the major gripe tending to be the slippery slope problem. Where something that someone dislikes not only gets reused, but built upon, further straying from that persons personal ideal for the setting.
The most often seen complaint is along the lines of Steel Watch and Factories can lead to more industrialization and thus gunpowder and then firearms and then all of a sudden the setting isn't fantasy swords and magic but people running around with AK's and flying fighter jets and then it becomes laser rifles and space ships...
That said, this slippery slope argument doesn't hold a lot of weight, due to the fact that it's not even guaranteed that anyone will ever re-include anything like that, let alone build upon it. Also, future writers always have the potential to write something out of the setting (Stuff like "Mystra didn't like gunpowder, so she used magic to make it inert and thus useless" because magic and gods are great sources of plothole filler and retcons).
Also, it's far more likely that more drastic changes to a setting might instead be replaced by a new setting that better fits it. Much like there's the 40k setting to provide a futuristic theme for the Warhammer universe without affecting the OG swords and magic Warhammer setting.
