Is your argument that The Forgtten Realms is a nonsense non-setting where anything goes and everything is fine and acceptable at any point? Is your argument that "if a story were to suddenly be written where the city of new york exists in its entirety in Faerun, that would feel totally in line with the setting because it's a setting where in theory WotC just has to approve it for it to be canon"?
The consumers feelings do in fact matter because who do you think the setting is being made for? If BG3 had been a failure and people had been complaining about how it didn't fit the pre-established tone of the setting, do you think WotC would stick to their guns and say that it's absolutely canon? No, they'd ignore its canonicity.
Your claim ignores the fact that the setting has in fact existed in a particular way for a long time leading up to the release of this game. People have experienced the setting for decades leading to this point. It's been a hodge-podge yes, but still has had some degree of consistency in flavor. So when things are introduced arbitrarily that go against that flavor, people are well within their rights to say "that doesn't fit with what's been established before." Then WotC or whoever are free to say "you're right, we'll change it" or "well that's what the setting is now". The implication of your argument is that fans should just silently take whatever they're given as though WotC is doing them a favor. That fans should just pretend the setting as it existed in the past doesn't exist and should have no bearing on their feelings about any decisions made in the present. That is not how people work, nor is it how businesses or even creative projects work. If George RR Martin were to say in the next Game of Thrones book that it's canon that cell phones exist and people travel around on motorcycles, then that's as much canon as anything going on with BG3 and the forgotten realms. You could argue that "since he's the author and IP holder, he's entitled to make the setting anything he wants" and it would be just as valid as what you're saying about WotC. People would still have every right to complain about it, dislike it, and stop buying the books.
Furthermore, this particular line of discussion began with KillerRabbit claiming that Larian don't particularly like the setting of the Forgotten Realms as it existed and so by act three they were introducing thinigs to it that did not fit in the setting as it had been established previously to make it more to their tastes. So again, the point isn't that WotC can make anything canon. The point being made was that Larian did not like the forgotten realms as it had existed before they were allowed to influence it, and as evidence, KR pointed to a number of things Larian included that run counter to the setting as it had previously been presented. It doesn't matter that those things can be allowed if WotC says they are, what matters is that they weren't there before and Larian added them in because they were unsatisfied with the setting as they had recieved it. The argument is that if Larian had been happy with the setting, they would not have added the things they did. What is and isn't canon is another discussion.
