I'm wondering if Canon took a long hard look at the Tokina 150-600 and decided that they had to go back to the drawing board to come up with something competitive.I would love for that to be the reason for the delay in the 100-400mm update. Despite the Tamron's obvious flaws I would think they've carved quite a little corner in the not-insanely expensive supertelezoom market.
Do I think that is the actual reason? Unfortunately no. I think Canon is just content with the offering they have so far and have yet to feel the pinch/need to respond yet.
Canon's lens development cycles (except for maybe their 18-55 crop kit lenses) are long and carefully planned out. I'd be stunned if they'd go back to the drawing board on a lens that was pretty far along just based on one competitive offering.
I suppose it might happen if Nikon really hit it out of the park on a pro staple lens like a 24-70 or 70-200, but redirecting / setting new goals on a lens is a massive disruption to many people, materials and dollars that already have momentum in one direction. I really doubt Canon does this very often unless it's very early in the development effort.