If the focus ring was at the other end of the push/pull barrel so it didn't get turned accidentally, and if the IS was worth anything, it would be a far more pleasant lens to use.
Have you used the 70-300L? The design places the focus ring next to the body, the zoom ring further out. That's reversed from other L lenses, and means your hand (well, mine at any rate) reflexively grabs the focus ring when intending to zoom. A real PITA, and if the new 100-400 has a similar design, it would be unfortunate.
I own the older Tamron super zoom (200-500mm). Its focus ring is placed where it is next to the camera's body...and I kind of like it that way! When used on a monopod or tripod, it seems a lot better that way...but certainly different from my Canon lenses.
I own a number of Canon L lenses - which started out with the Canon 28-135mm 'back in the day' which has the focus ring close to the body, and the zoom ring further away. (I've since sold the Canon 28-135mm)
Currently the 15-85mm and 70-300mm L are among my 2 favourite lenses (mainly used on my 7D) - and I actually prefer the configuration of focus ring close to body and zoom ring further away. The Canon 70-300mm L won for me over a number of other options (a few years ago) - because of it's IQ and size/weight. I did consider the 100-400mm... but that extra 100mm of zoom wasn't 'worth it' for me (I understand it might be for others).
I have used the 100-400mm a number of times, i.e. using friend's lenses, borrowing, etc. I became more accustomed to the 'push pull' after some time than I thought I would, but still generally by 'instinct' prefer a zoom ring, and a zoom ring nearer the end of my lenses. Having said that, I know it would be a different story if I had only got used to lenses with the zoom closer to the body (eg if I had owned the Canon 24-105mm L instead of the Canon 28-135mm). But hey, the Canon 24-105mm L wasn't around at the time I got into photography!
I have owned 3 x Sigma UWA lenses (and initially found it mildly inconvenient on each that 1) the zoom rotates the other way AND 2) the focus is at the end and zoom ring is nearer the body. Thankfully I pretty much have a 'mental switch' in my mind now, so when I put my Sigma 8-16mm on my camera, I somewhat reflectively know it's on and my hand/mind know what to do (eg in the dark).
Horses for courses... but most importantly, enjoy your gear and take great photos. I trust that Canon will have done their research (& design) well; and if they do
come out with a 100-400mm L II (or similar) that they will come out with an awesome lens. However I doubt I'll be buying one... as the 70-300mm L meets my needs very well.
Best wishes all...