Specifically, if you use the 16-35 f/2.8L II to do event photography, such as crowded dancefloor shots at dim receptions or indoor sports - all of which will likely require at minimum 1/100 shutter speed - the 16-35 f/4L IS will be virtually unusable in this situation compared to the 16-35 f/2.8L II.
I don't understand this line of thinking. Were these events virtually undoable during the 5D2 era and completely untenable when the 5Dc was in its heyday? Between the high ISO capabilities of modern bodies and noise reduction algorithms like DxO prime, nobody really needs
f/2.8 outside of depth of field concerns, some indoor sports, and maybe the astro people. Heck, I've shot indoor table tennis tournaments where I felt comfortable stopping down to f/5.6 on a 70-200ii for the extra dof while still being able to maintain good enough shutter speeds to stop action without the noise getting out of control.