Each to his/her own. I am feeling feisty today and I will respond.
I like zoom versatility. If nothing else, it makes it much easier to locate a bird in the foliage quickly and then zoom in close. It also eliminates a lot of need to "zoom with your feet," plus it saves a lot of the fiddliness of putting on and taking off teleconverters. In some situations, this can be a thing.
The OEM foot is not great, but once in the field and actually using the lens, I found am not bothered. The hood is fine.
As for IQ, I have evolved over the years from serious pixel peeping to the school of thought that thinks "sharp enough" is sharp enough. I don't go as far as Ken Rockwell with this stance, but I think he is correct when he asserts that many people compulse way too much about "sharpness" and hence talk themselves out of using lenses that are just fine and maybe don't break the bank. Can one tell the difference between the highest quality fixed focal length tele and a budget zoom? Of course, but it really only matters if you are cropping heavily and/or making huge enlargements. For what most of us use our cameras for these days (online reproduction), a "good" consumer zoom (like the 400-800) is plenty sharp enough, and this before even bringing up the subject of judicious post-processing sharpening. And yes, whatever CA there is will almost surely be eliminated once DxO creates a lens profile for the Sony zoom.
As for size, the 400-800 weighs 1.25 pounds less than the Sony 600 f4. To me that's significant. Would I rather have a 600 mm f4 that costw 4x as much? Heck yes. We are talking apples and oranges here.
I own a few really ridiculously sharp tele lenses, including the Panasonic Leica 200mm f2.8 for M43, the Olympus 300 f4 for M43, and the Nikon 600mm f4G VR. I do revel in how sharp the images are coming straight out of the camera. But this doesn't stop me from owning, using, and enjoying lenses that are more versatile even though they are not quite as sharp.