Never. Unless explicitly approved as such by the original creator nothing supersedes the original source as canon. In the case of Middle-Earth no amount of movies, games or anything else will ever supplant Tolkien’s books as canon (especially something as kludgy and careless as Peter Jackson’s films).
In the case of Star Wars the films will always be canon (as much as I wish it weren’t so with the prequels). As far as Disney now jettisoning off all the old EU material – it had to be done, frankly. The EU was overfilled with material, often contradictory (and, to be honest, frequently of poor quality). Lucasfilm were basically approving anything with no regard to quality or continuity control. It would have made the task of continuing the film series all but impossible. And since they do not wish the go the goofy route of having multiple timelines, alternate “universes” or whatnot, casting off the old EU and starting fresh is the only logical move they could have made.
Personally speaking, as much as I hate losing something like Shadows of the Empire, this relaunch is part of what’s actually made me somewhat interested in Star Wars again. That said, Disney says that it will, via the new Lucasfilm Book Group, keep a tighter control over the EU canon than what Lucasfilm under George Lucas did, but they do not, apparently, rule out the possibility of incorporating some old EU stuff into the new official canon.