I dislike the term "canon" because it carries some hefty religious connotations with it, and to me "religious" runs into the risk of "dogma". While the concept of heresy is rather amusing in-universe in
40K, I find it less amusing out of it, and I've had enough horrors in dealing with canon of other universes in the past to want to deal with it again. I personally doubt you would ever get people to agree on what is or isn't canon - people have too many differing opinions, likes, dislikes and so on. And as near as I can tell the individuals at
GW and their spinoffs have different opinions on canon as well, given all the quotes.
GW itself doesn't seem inclined to take a specific stand (the way, say,
ST and
SW have) and that may actually be wise siince it won't be possible to make everyone happy.
Don't get me wrong, I think it can be worked out, even if it wouldn't be an easy thing to do and it would require cooperation and even some compromise. But I don't believe in one absolute truth and I doubt anyone would get anyone else to agree.