Why do people keep thinking that a 24-70mm f/2 is even remotely likely?
Have they not held the already behemoth Canon 24-70 2.8 mk2 or Tamron 24-70 2.8 VC, both which require 82mm filter threads just to sharply render that zoom range at 2.8?
They are modestly sized lenses. Not big at all.
Have they not noticed that the Sigma 18-35mm f/1.8 DC is only a 2x zoom, not a 3x, and that it only needs to worry about a largest diaphragm of 35 / 1.8 and an APS-C image circle? (HUGE difference from 70mm f/2 with an FX image circle!!)
18mm f/1.8 on crop is a harder thing to get to than 24mm and f/2. This is because Sigma's crop lenses don't reach into the body like EF-s lenses do, so they have to have the regular back focus distance of a full-frame lens. This makes 18mm much more strongly retrofocus than 24mm, and f/1.8 is still faster than f/2. I doubt much in the 18-35 is driven by the aperture at the long end, and 70/2 is still only 35mm, which isn't very much aperture. My 85/1.8 is silly tiny and has 47mm of aperture.