But 200mm/2.8 is 71mm of the entrance pupil no matter what mount is used at the back of the lens. That's a lot of glass, needing a lot of support. What makes you think that the EF-S version would be considerably cheaper and/or lighter?
The after market already has $1500 70-200 f/2.8 offerings. I don't see the motivation for Canon to compete in that arena.
One of the main issues a lot of people have with the current 70-200mm f/2.8 IS II is that it is too heavy to comfortably carry through the day, and it costs $2500.
By adding a EF-S F/2.8 70-200 or EF-S f/2.8 55-250 IS etc, Canon does a few things:
yea thats the thing. the only way to make it lighter is to use fewer elements and inferior construction materials, neither of these things would amount to a serious contribution in my opinon. Witness the $1250 Sigma, which is only two ounces lighter than the Canon. The "cheaper" aspect might come from lesser build quality and inferrior glass at least in the rear elements, as the the image circle seen by the sensor does not have to be as large as in a FF.
So the expectation of 70-200 that is lighter
remains a fiction. one that is cheaper
could be acheived, I suppose, but I can't imagine settling for anything less than what is already available in the after market. Canon would have to basically offer a me-too lens competing directly with the current after market, or produce something even lower quality, say at $799 which frankly I can't imagine would be attractive or succesful. In fact, as I mentioned in an earlier post, the market has already established that a low cost 70-200 in the $1k region has to be FF compatible,. ergo, EF-S is'nt going to happen.
so -- what is being asked for here already exists: The Sigma. it isn't lighter but it is cheeper, and sports all the IQ features that are associated with the lower cost, compared to the $2500 canon: inferior AF performance, CA, edge-to-edge sharpness, flare control and build quality. and its FF compatible.