Yup, Canon doesn't want to eat their own lunch. Problem is that they're then exposed to others eating their lunch. Canon is never very aggressive here.
LOL. People who speak of f/5.6 zooms or primes 'cannibalizing' supertele sales are rather clueless. The superteles are on a whole different level of IQ, are 1-2 stops faster, and have better AF. Pros who need a supertele generally know it, amateurs often move up from the 100-400L, so that lens is sort of a gateway. Even with a big IQ boost, a new 100-400L won't beat the superteles. Canon also prices their lenses to ensure profitability.
As for 3rd party competition, that's been there all along. For some lenses, the quality has gone up (though Sigma's QC issues apparently continue to plague them)...as have the prices. The Tamron 150-600mm is a decent lens, with similar IQ to the current 100-400L (similar in the overlapping range, similar at the long end to the 100-400 + 1.4x). A 7DII/X with f/8 AF will make the 100-400 w/ TC quite feasible as a much smaller and lighter choice over the Tamron. The Sigma 150-600mm is even bigger and heavier than the Tamron.
Will we see an updated 100-400L? Yes, I think we will. Likely in late 2015 or spring 2016. Canon is good at extracting profit from consumers. So...release a 7DII/X now, the new action-oriented APS-C body will spur sales of the 100-400L. Some of those who have one already will buy the 1.4x because of the f/8 AF. A year later, release an updated 100-400 to get people to upgrade.
It might make philosophical sense to release a new 100-400 along side a new 7-series body, but it probably doesn't make fiscal sense. The economy isn't terribly strong right now. Spreading out the launch of 'big ticket' items targeted at the same market segment seems logical.
I totally agree that the 100-400mm is more of a gateway drug to the big whites. Not going to cannibalize sales, only going to give people a taste of what they want.
I think you are wrong about the third party lenses. The Tamron is 1/2 the price of a 100-400mm plus 1.4x III and only weights about 330 grams more. I think Canon has missed the boat with the 100-400mm upgrade. I would expect Canon upgraded lens to be in the same ballpark price wise as the Nikon 80-400mm. Tamron is going to eat both their lunches. It will also eat Sigma's lunch if the weight and price of the Sigma 150-600 are as rumoured.
Canon can improve the 100-400mm but I don't think they can make it price competitive with the Tamron.
I already know several people who have upgraded from the 100-400 to the Tamron.
Personally I got tired of waiting for the upgraded 100-400mm and added the Tamron to my kit to replace my Sigma 150-500mm.