I agree that the whole notion of "canon" is faulty, but will take it a step further: It makes no sense for a world created by a single author. The whole purpose of assigning a "canon" is to have an agreed upon body of lore and stories for a shared universe - that is, a world with no single author, that multiple people make stories in. This, in turn, guides future stories.
So it kind of makes sense for Star Wars or Star Trek or Marvel or the Forgotten Realms, not so much Middle-earth.
As for The Silmarillion, I'm reminded of that quote from Robert Frost or Mark Twain (can't remember who): "Poets don't finish poems, publishers do." Or some such. In this case, Christopher Tolkien was the publisher/editor/compiler, but not the author. The poetry was all JRR; Christopher just put it together in a cohesive manner for publication. And thank the Ainur he did!
And perhaps more importantly: The stories of the Silmarillion were the heart of the "matter of Middle-earth;" they were Tolkien's truest love. To disregard them as "non-canonical" is even more non-sensical than imagining a "canon" for a world created by a single author.
I think a more meaningful distinction for Middle-earth is not canon vs non-canon, but Tolkien vs non-Tolkien. In fact, I think that's the only meaningful distinction. There's Tolkien's works--in whatever form or fashion, or state of completion--and then there's everything else, which are adaptations, no more or less. Whether or not one prefers the adaptations is beside the point; they aren't actually Tolkien's work, just "takes" on his work.
