Lucas gets to decide the Canon because he created star wars. We can call him out on the fact that the prequels were written after the fact and are not true to the star wars films we all grew up with.
But as an author he could change his conception of star wars later on and recast the universe in that new mode of thinking. He changed as a man and a filmaker over twenty years.
I think he has a right to do what he wants with star wars as long as the original films are given the respect fans demand.
He has a right to change his mind creatively and try new things and ideas. Sometimes risky ones that get him condemned by oot purists.
But as we are all talking about star wars. Which is a fictional universe i think we need to stop and think before it leads to personal insults and attacks against people. I mean that is just silly to get angry over a piece of fiction.
To me though anyone who thinks the prequels are not a very deliberate recasting of the star wars universe which Lucas started with the special editions is blind. IMHO. Not a personal attack or condemnation just an observation.
And Vader is not hayden is right about the oot being the one true canon by which all other star wars should be judged. It is not really a personal Canon. If it was like me you could add the zahn trilogy and other bits and pieces of the sequel era EU as you liked. There are some excellent parts to the EU and then there are some painfully bad parts.
To me if you only care about the 3 original films it is a bit closed minded because you never really see the characters move on and grow beyond return of the jedi. And since Lucas denies ever having in mind a sequel trilogy the EU is a second best option. Therefore the sequel EU to me is canon. Because Lucas gave up on the fans who wanted to see more adventures of luke skywalker as star wars was always meant to be. And changed it into the 1-6 episode story crap of hayden vader.
I mean who cares for the retcon prequels and Hayden as Vader. If the prequels had to be made at all at least they should have been written as true backstories and not changed to fit Lucas whimsy.
Just like there is almost nothing substancial about the films that could not have been achieved in the 1980's or now using old school methods of filmaking models, matte paintings etc. 90% of the prequels is CGI overloaded 10% is a paper thin story a kid would get a D- on.
So the strength is in the visual sense here which is Lucas strength but the storytelling suffers and takes a back seat.