For what its worth, I would probably only pay $2000-2500 for a 35-85mm f/2 that weighed a little over 2 lbs and was comparable to their 24-xxx zooms in image quality (the 24-70 ii is about $2050 currently on amazon). My wild and speculative guess is that if Canon were to actually introduce such a lens it would be more than double that price, so its kind of a moot point for me. I would still be interested in seeing design info though.
Yeah, I don't think its a technical limitation, it's more an R&D time limitation that keeps them from making f/2 zooms. As others have mentioned, you'd need a lot of glass and large filters, which by weight and filter size eliminates a lot of the market. Then you have to recoup the cost of R&D, and the cost of all that glass, so your starting point price is already higher than $2500+ I'd bet. Then, because its expensive, fewer people are in the market. Forum kings will complain they can have a 35L, 50L, and 85L for the price, and they are all faster than f/2! So your market grows smaller, and cost goes up. Assume it's a $4000 zoom; who is really buying it over the 24-70 f/2.8 at half the price? Those that NEED f/2 also probably NEED f/1.4 as well.
It's also looking to solve a problem that basically doesn't exist, and that pros likely won't pay for. They'll carry a 35L and an 85L, or a 24-70 and a prime instead. Whereas, with the 200-400, getting a 200-560 zoom that is super high quality and doesn't necessitate lens changes in bad environments is something a pro will pay for. If twice as many paid for it, the lens might cost $8-9000, instead of $11,000+.