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.
I think we're in agreement here. The response I'm talking about is their response to slowing sales of their current offering. Not necessarily from a Sigma, Tammy, or other 3rd party lens offering.
However, it would be sweet to see what Canon could do with a somewhat similar zoom range as the Tammy (something like 200-500mm, for example) that doesn't cost $12k. Maybe that's one we can look forward to seeing at Photokina 2020.