There are pictures on Bob Atkins' website of the 200-400 with a 7D attached:
That DI filter isn't coming out with the lens mounted...
I'm utterly amazed that any of you are stunned at this design decision on the 200-400mm and even pretend that this might be changed. Have you looked at Canon's other super telephoto lenses? Lenses that share this exact same DI filter location (under the pop-up flash) are the 200mm f/1.8L, 200mm f/2L IS, 300mm f/2.8L IS, 300 f/2.8L IS II, 400mm f/2.8L IS, 400mm f/2.8L IS II, 400mm f/4 DO, and probably several others. Is it slightly annoying when using the 7D? Yes, but usually I know if I want to use a polarizer or not when I start shooting.
Other than the filter I could imagine accidental body/lens detachments happening when changing focusing modes and MFD.
like the switch on the left side before the bulge.
AF -> PF -> MF -> Lensinthemud
Last I checked, just pressing the lens release button doesn't immediately drop the lens off the camera. Some amount of turning was required. Maybe it's not an ideal location, but the AF/PF/MF not an often used switch. Canon could remove this switch and I wouldn't notice. However, again this design is shared with the other new super telephoto lenses. The switch location is identical on the version II 300mm and 400mm. It's only a little farther away on the version II 500mm and 600mm.
Side of the 300mm f/2.8L IS II:
I can see where the 1.4x might be causing them some design headaches since it's new to their still photography lens line-up. Perhaps there's some design challenges there that were not expected. However, including a switchable extender in a lens is not new to Canon: http://www.bhphotovideo.com/c/product/424578-REG/Canon_YJ20X8_5B_IRS_YJ20x85BIRS_2_3_20x_ENG_EFP.html
*shakes head* Honestly I think people on this forum really need to stop just trying to invent problems and letting their imaginations run wild without doing a few moments of research.