Reading at the reviews posted, Canon improves the overall sharpness (especially in the corners, this was a weakness of version II), distortion and flare control. When I hear about fast lenses for the landscape photographer I believe it's irelevant as such photographers will shoot between f8- 16 ?
The amazing sharpness could be the reason for the upgrade. Anyway, who shoot landscape or architecture with such aperture? Or is fast lens a better designed and constructed pro lens and that makes it better ?
Don't get me wrong, all of the 14-24, 24-70 and 70-200 lenses are excellent, but they require a 3 lens load out if you're interested in covering ultrawide-normal-tele lengths.
Canon offers a way to bridge that same span with only two lenses, which can make shooting an event, or travelling, massively easier for those people that work with two bodies.
I have a 16 /35mm 2.8 (series 1) lens and a 16 / 35 f4 IS lens each of which I use on either a 5D3 or an 80D. Of course I would like the new version 3 lens but on crop cameras there is not much difference between version 1 and 3.