Your alternative POV is something akin to the canon can only be discovered or defined in relation to something else. I am using a more focused literature one.
Whereas mine is the canon 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 understand what your viewpoint is, I simply do not see why you would view all sides as canon when one obviously wasn't written by the creator of it.
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, I suppose it's down to personal preference.
And Ben, you're still using an argument of authority.