if you have sources to tell you that there are prototypes.
why don´t they tell you at least if it is a push-pull design or not?
Personally I doubt CR has any good inside information about Canon :-> though there's a beta tester around here that often comments on current prototypes and seems to be very reliable...
... but concerning your question, the answer most likely would be that there are several prototypes with different characteristics and designs, and only God and Canon knows which one they'll select for production.
We’re told that the current 100-400 still sells at an amazing clip and outpaces the 70-300L by a 4 to 1 margin.
Now *that* is a surprising and interesting piece of information, if it's true and since Canon is not in the habit of replacing best-selling lenses without a good reason it means one of these two things:
1. (less likely) The release of a new 100-400 will take some time after all because there's no immediate market pressure
2. (more likely) The new 100-400 will be much more expensive, i.e. generating more profit for Canon, and will be an "upgrade" to the existing lens rather than a replacement on the same level - more like 24-70 mk1 to mk2.