I had to create an account because I couldn't let the discussion end here.
Saying everyone has a valid viewpoint in a fictional universe with established facts is a logical paradox, this is a truism which people seem to have overlooked. If everyone's opinion is valid then the opinion that everyone's opinion is valid negates itself by the existence of an opinion contrary to this axiom. By your own hand you've just broken your argument, the simple fact is that everyone's opinion cannot be valid when it comes to non-subjective matters (a brick is a brick, as opposed to I like sweet drinks more than bitter). My opinion on physical equations isn't equal to that of an established physicist, it is either right or it is wrong. This isn't mentioning either that allowing all opinions completely destroys discussion even more so than canon wars (e.g. "Khajiits aren't real", "yes they are", "no, in my opinion they aren't", "oh, OK, goodbye").
There needs to be an established authority, which most people deem to be Bethesda and what they choose to incorporate into their games, such as some of Kirkbride's work and stuff ffrom the books. Now of course I'm not saying people can't have an opinion, but it should be widely acknowledged that such opinions are incorrect to promote proper discussion without bringing subjectivity to the table.
I understand what you're saying and personally I'm not an ardent follower of MK's standpoint. I was just explaining his reasoning.
As mentioned, the reason he released C0DA was it was his attempt to end the arguments over what is and is not canon. According to him, if you have a theory or belief about something in TES, then go ahead and believe it. No one should stop you from building your own world using your imagination. Its hurting nobody. People call it "headcanon".
Its nice sentiment, but of course it fails when a community gathers to discuss a certain subject and everyone begins to add their own headcanon. A discussion about the origins of the Akaviri can go in 50 different directions and really all you get is a bunch of theories. If that's your thing, then cool, and there are forums dedicated to people writing their own lore texts, theories etc.
But there are still many of us who are more interested in disecting the facts have been revealed by an established authority. Many draw the line at Bethesda = canon and nothing else. Most of the time, these two groups can't really have a discussion because canon will come up at some point.
Whether or not MK has the authority or right to declare the canon of a series irrelevant is another issue. Some people loath the idea of Bethesda owning the canon and feel that they have every right to be involved in it. Some of these people believe that adhering to an established canon from an authority is oppressive and words like "intelectual property" or "corporate" come up. What these people don't understand is there are still many fans who are not interested in fan-theories and prefer to have an "official" canon.
Again, the "no-canon" people are free to believe what they want if they are not hurting anyone, but they need to understand that nobody else is under any obligation to accept their headcanon.