I know I'm in the minority, but I sure hope the 100-400 replacement is a push-pull zoom.https://www.google.com/?gws_rd=ssl#q=advantages+of+push-pull+zoom+lens
At least two of us in that minority...
I have wondered about the pros/cons of push/pull zoom and found this link, which mentions the 100-400
Interesting discussions with personal preference perhaps a factor. I think the push/pull design has made way for the rotating ring mostly because of recent advances in autofocus. And just maybe the rotating ring may cost less to make??
I have a strong preference for push-pull (but only in a lens like the 100-400, I would find it undesirable for a shorter lens) for two sports related reasons.
First is for getting around lens restrictions at many sports venues. It's often based on the physical length of the lens, not the focal length. At 7.4" collapsed and sans hood, it's really quite petite for a lens capable of super telephoto focal lengths. So at a place with an 8" restriction, it's the only way to get out to 400mm without TCs. The 400DO clears a 10" restriction, and you could probably get away with the 400/5.6 as well (it's 10.1").
Second is just the amazing speed you can go from 100-400mm. At 100mm you can see between 1/3 to 1/2 of an american football field from the stands, depending on where you're sitting; which is great for scanning the field for action, then with a flick of your wrist you're at 400mm with just a couple players in your FOV. Try that with any of the 70-200s and a 2xTC. You can probably do it in only about twice the amount of time if your hand is positioned just right on the zoom ring where you can do the whole range without repositioning your hand, otherwise you're probably looking at 5-10x as long.