Any zoom will suffer from the limitations you listed compared to a prime. In your opinion, how bad exactly is the 24-70 II to warrant a new zoom to cover a subset of that range for improvements?
My take on the 24-70 II is it's so close to perfection, that it seriously makes you wonder why you should bother with primes in that range. Yes, a small handful are faster (with a narrower DoF), may have less vignetting in the corners when stopped down to f2.8, and can provide more transmission. But make another f2.8 zoom, and no matter how perfect it is, its unlikely to better any of that small selection of primes on those areas listed, and in overall image quality, there's not much room for improvement over the 24-70 II.
For such a small zoom range, especially one covered by a near perfect lens already, why bother making it? How big is the market?
If you're talking about a range not covered by any near perfect zooms yet, then yes, its quite likely to be worthwhile. A theoretically perfect 16-24/2.8? Same sort of zoom range (about a 1.5x zoom) as your mentioning. However, it's still a bit short for most peoples taste - after all, if you've got all the expense, size and weight of a zoom instead of a prime, it'd be nice to not have to guess which end of the zoom its at when you hold the camera to your eye.