It will all come down to the optical quality. It has to about match the 100-400L at 400mm and f/5.6. If it's substantially worse than that, and up-res of the 100-400L might beat it.
I kind of disagree, simply because this goes to 600mm. If it can match the 100-400 at f/5.6, then thats a really impressive feat, but, I think its equally important for it to be as sharp at 60mm as the 100-400 or 400 prime would be with a TC on. Cause if it is, then the fact it does it natively could be another leg up.
But really, the key is gonna be price. If its around the same price as the 200-500, then the range it gives might cause birders to go with it over the 100-400 (IS less important, and 600mm more critical than 400mm). Especially if a new 100-400 comes out and raises prices. If it costs more like $1300, then I can't see as many going with it