...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.