Unfortunately I think some are so focused on sharpness and sharpness tests these days, they forget that sharpness is only a piece of the whole pie. There are other elements, such as max aperture, bokeh rendering, flare, color/contrast, focusing ability, falloff, ca, size/weight that are equally important.
thus, I believe some are feeling the 16-35 f/4 IS is a replacement for the f/2.8 II since the former is sharper than the latter. But as you note, it is definitely not a replacement as many would never give up f/2.8 for a bit sharper corners.
Lenses all have a tradeoff, and I wish more would look at the big picture rather than honing in on sharpness above all else. It is important, but many other things are important too.
OTOH, in UWA territory I bet far more care about lowered LoCA and greater sharpness and IS than f/2.8. (plus this is smaller and lighter than the 2.8 and who says it won't have better color and contrast and flare resistance, it has even more modern coatings and design).
Per the MTF it looks like the 16-35 f/4 IS will have enhanced sharpness/contrast over the 16-35 f/2.8 II and the smaller front element will likely help reduce flare on top of coatings (look at the 24mm f/2.8 IS if you really want to see reduced flare, btw). Initial reports also have indicated better corrected LoCA.
But, if you are shooting people such as events (especially indoors), generally all of that is still no substitute for f/2.8. Shooting people you want to be at 1/100 (unless you intend to introduce motion blur purposely to indicate motion), so Image Stabilization is of no use and f/4 makes it near impossible to hit that shutter speed in a dimly lit event without cranking the ISOs up to a level that will damage the picture far more than any of the lens' improvements. In fact, sometimes even f/2.8 is not enough and it is necessary to use my 24 f/1.4L or 50 f/1.2L.
While many will use this as a landscape lens, I think Canon will come out with something even better for landscapers in the future as this lens still does not quite compete with Nikon's 14-24, moreso with the 16-35 f/4 VR. I would argue that 14-24 is a more useful range for landscape than 16-35, plus the Nikon does f/2.8 while remaining sharp across the frame.
Why in the world do you not think this will be competitive with the Nikon 16-35 VR??
And not every landscape shooter prefers the extra 2mm on the wide while giving up 11mm on the long end, that can mean a LOT, LOT more lens switching, plus you don't have to deal with Mr. Bulbous and the filter difficulty issue. And again, IS would be much more help than f/2.8!![/qu
If you reread what you quoted, you will note I stated it *will* be competitive with the Nikon 16-35 f/4 VR. What it is not competitive with is the Nikon 14-24 f/2.8. Which is why I think Canon has something better up their sleeve... They are not going to leave that market on the table all for Nikon.
I understand that not everyone wants a bulbous element (I do not), but some desire the ultimate performance which for UWA has thus far required a bulbous element or otherwise massive front element. That is not to say the 16-35 f/4 IS is bad, but it is still f/4.
16-35 range is very useful to wedding/pj because it works in very cramped spaces while still being able to do 35mm f/2.8 portraits.
I guess, although many use 24-70 2.8 more and even 70-200 2.8. It depends.
But what are you complaining about? You say you don't care about sharp edges and so on so why is the current 16-35 not good enough for you then??
I think the current 16-35 II f/2.8 is good enough for my uses, which is why I responded to mack in the first place (reread the first reply to the OP). Sharper corners would be nice, but I would not give up a whole stop for that as they are sharp enough for my uses as-is and the loss of a full stop would be much more destructive to my photography than sharpness gains in the corners.
16mm is actually useful for tight indoor spaces when 24mm is not wide enough or when you are trying to take a picture of a single couple on a busy dancefloor. While 16mm does introduce some distortion, if you keep the people in the middle of the frame you can still take a nice looking picture that would not be possible with a 24-70. And, if you get the opportunity to zoom in, you can get some bokeh still with 35mm @ f/2.8. Not a ton of out of focus area, but a ton more than you would see at 35mm @ f/4.
So for some events it is nice to put a 16-35 II f/2.8 on one camera and a 50L or 70-200 f/2.8 II on the other camera. Because of the shutter speed required to avoid motion blur, f/4 simply will not cut it in this situation. And hence, why the 16-35 f/4 IS is not a replacement of the 16-35 II f/2.8 - it is simply a different option that appeals primarily as a landscape lens.
I see 3-4 markets:
1. Budget: 17-40 f/4L (this may be a temporary category until Canon goes through leftover stock, though).
2. Landscape: 16-35 f/4L IS - Sharp and nicely corrected, but can only do f/4.
3. Event/PJ: 16-35 f/2.8L II - Not the sharpest in the corners, but f/2.8 allows for faster shutter speeds in events and better subject separation at 35mm. Since this type of photographer would be mainly shooting people, corners are less important as you can't place your subject there anyway due to distortion.
4. Extreme performance: 14-24 f/2.8L - Does not exist yet, but I think something like this will come out from Canon soon to counter Nikon's 14-24. This category is for those who want the fastest and widest zoom regardless of weight and front element size. Probably not the most practical for long events and hiking trips, but to be used when you want to be using the fastest and widest rectilinear zoom even if it will be bulky, heavy, and potentially require an expensive filter system.