Then, my friend, you have a subjective conception that only what comes from Oda's head can be considered "canon".
Unless you know what's in Oda's contract with Shueisha, based on the "status quo", the property of One Piece manga belongs to Shueisha. Oda, being the creator, gets compensated. But there should be legal barriers that stop him from simply deciding one day to say "fuck it, I'm gonna take One Piece out of here and go publish it somewhere else".
You can't even say that EVERYTHING in One Piece is "Oda's idea", considering that is possible he took advices or changed things in the story, influenced by someone else's idea or suggestion.
If Oda were to die, or be incapacited to continue his work anymore by other means, and Shueisha keeps publishing it and stating "this is oficially One Piece material", you can say "But I dont CONSIDER it canon".
Because "Canon" is a subjective matter, that's why disregarding other media is wrong. It may not be relevant to you, but it is to someone else, and they have as much entitle to their tastes, as you have to yours.
Rights do not matter, creator matters, once again bringing up something irrelevant.
You are literally talking about business matters, i am talking about the creative source.
Everything in One Piece is something Oda decided on. How his process is, how much input and feedback he considers, how much inspiration he takes from something else, does all not matter, he is the creator, it´s that simple.
It´s not, there is a definition, hence why i said, look up the definition. The word itself comes from two distinct contexts, one is the Bible, and only taking certain scriptures as canon since others were added later, and also Sherlock Holmes, once again distinguishing everything written by Arthur Coyle himself to be canon, and every novel and adventure written by other writers afterwards as not.
This is not a matter of opinion or subjectivity.
Of course you can spin your own impression in your mind, does not change the fact that it is not intended to be like that.
Most of the time, the creators can extent canonity to other mediums with their license if they so want to, George Lucas gave some employees of Lucasfilm back in the day canonity license so to speak, so they are on the same level as him.
And while Oda stated the anime team does a good job, he never extended such courtesy to the team behind it, and explicitly disconnected the manga from the anime.