Will watch the video in a bit, but I'm already on fanfiction's side since ALL fiction is effectively fanfiction and whether something can be 'canon' tends to depend more on the copyright status of a work than anything else.
For example, if were to write a brand new story about Detective Benoit Blanc from Rian Johnson's Knives Out except I placed him in ancient roman times (In which, he keeps the southern accent, of course) and publish it, I'd have some royalties to pay to Rian Johnson.
But if I were to do Sherlock Holmes, no one would bat an eye. Because Sherlock Holmes is public domain while Benoit Blanc is not. But the actual act of taking X character from a franchise you didn't make and placing them in a place they weren't intended to be in by their original creators.
And no one in their right mind would deny that the various adaptations, spin offs, and replications, the original Sherlock Holmes stories have been not only matched, but surpassed.
(incidentally, Neil Gaiman wrote what imo is one of the most elegant fix-fics. He didn't like the idea that Sherlock Holmes would go be a beekeeper, since that seemed out of character for him. So he wrote a story that contextualized why he did that was more in character, even though the context is one that Arthur Doyle would never in have ever written. It's great,
http://www.pages.drexel.edu/~ina22/200/The%20Case%20of%20Death%20and%20Honey.htm
And even 'original' works generally aren't. The act of imagination is not making up something wholesale new, but combining what is known in new ways. When someone made up the unicorn, they just imagined a horse that had a horn. When soemone made up dragons, they took aspects of lizards and bats and other animals to make a new animal.
Such it is with writing original fiction. George RR Martin himself admits that A Song of Ice and Fire is him retelling the true history of The War of the Roses but with dragons and magic, not dissimilar to how Tolkien constructed his cosmos after looking at Norse Mythology and throwing in his Christianity into it, and Star Wars is very blatant about how it's a mishmash of hundreds of different influences.
So I would argue that writers, however original they get, never actually stop writing fanfiction. It's just a matter of how self aware they are that they are alway, always, always combining and recombining constituent parts to make a new whole. The only practical use of originality as a metric is whether you can publish it without being sued.
www.resetera.com
