...a twist zoom instead of the push/pull that sucks dirt and moisture in everytime you slide it.
Is that Internet wisdom speaking, or do you have that problem with your copy of the lens? If the latter, your copy is likely defective and should be sent for service. If the former, did that 'expert' mention the fact that the 100-400L has dust/moisture seals under the switches and zoom/focus ring, and lacks only the mount gasket to be a 'weather sealed' lens like its push-pull cousin, the 28-300L?
I trust you're aware that lenses aren't hermetically sealed - any extending zoom design, whether push-pull or twist, 'sucks air' with every extension and expels it with every retraction. If you have a 'sealed' extending zoom like a 24-105L, etc., extend the barrel, then hold the lens up and look into the lens mount while retracting the barrel - the eye-blow will make you blink! The 100-400 moves more air, because of the larger internal volume, but making it a twist zoom won't change that. Making it an internal zoom, like the 70-200L lenses, would change that...at the cost of making it an >11" long lens, all the time. I say, "No, thanks," to that.
No it is not internet wisdom. I do have the 100-400. So far I have not had a problem big enough to send it in for service. There is some dust visible but does not effect image quality yet. I know others that have sold theirs for this reason before it got too bad so it is not just my
copy. Ironically I have have had more dust problems with the camera it is used on. Obviously something that moves that much air is going to move dust and moisture. I am far more cautious with this lens than with my other setups because of this risk. Unless they install a filter system the seal will not stop small dust and moisture particals. If the seal is tight enough to stop this without a filter then the lens wouldn't slide very well because the air couldn't pass through.
Now on the second part,(sorry I didn't make it clear enough) I was refering to the internal zoom like the 70-200 f2.8 where the air exchange is all inside because nothing externally extends. I'm smart enough to know that there is no difference simply because of the method used to extend it.
As the length goes, I feel it would not need to be >11". It should be doable by adding only a little more length than say the 70-200 2.8 IS II with a 2x tele installed. Yes I know there would be more to it than just add 2x optics. That is why I said a "little more" length. That should put it just under 11" which isn't a problem for me on a lens with that much range. A fixed length lens is a lot easier to keep balanced on a gimbal tripod at all focal lengths. Interesting thing is the 70-200 2.8 IS II I had with a 2x tele was just as sharp as the 100-400 in the center which is about all I'm concerned with at that focal length. Only time will tell what Canon will decide is best if any change comes.