Your alternative POV seems to be something akin to the canon can only be discovered or defined in relation to something else.
Mine is the canon, in fictional literature, can only be defined by "did the original author make it?" since, as it's their story, whatever their continuity is, it's the "correct" one.
I say the GT storyline can't be correct not because I don't like it, I really don't care either way about if I don't like a story or not, it's about "was this a work by the original author?"
Stories have their own internal continuity, but only the source material or the later additions to the source material by the original author should be considered the canon of said work.
The movie Sherlock Holmes: Game of Shadows is not a canonical work as it was not created by Sir Arthur Conan Doyle.
I might be incorrect about your viewpoint, but it seems to be that everything is equally canon in relation to each other.
I don't see why we should accept the facsimiles of fanfiction writers as equivalent to the original author's works and changes
It is not authentic, it is a facsimile.
Facsimiles should not be used as canonical material with regards to when the author (or someone they choose) is actively making new things and setting down "new canon"
Anything which goes against it is essentially a movie compared to the book.
If we go by your own definition of "In comparison to something else", then everything Akira Toriyama or Torayato haven't made is as non canon as their own work is in relation to it.
If we go by the "Only Toriyama or Torayato are canon" definition, then everything not made by them is non canon.
As for the question "why" should we take one over the other, simple. Do you want the authentic or the facsimile?
And Ben, you're still using an argument of authority.