Here's why I'd rather have a 24-70/2 than a 35-85/2.
A 35-85/2 is a "bag-o-primes". I have a 35/1.4L and an 85/1.8. So, at first glance, it sounds great.
But, it doesn't replace a standard zoom because it doesn't go wide enough. So I'd still need a 24-something (24-105, 24-70/2.8, whatever).
A 24-70/2 would serve the purpose of both lenses and the difference in perspective from the change in location to maintain constant framing between 70mm and 85mm is subtle, as is the difference in subject isolation.
Of course different lenses suits different shooting styles and purposes.
However, if 35-85 is a bag of primes then so is 24-70 or even 24-105.
It is all about who finds which FCs useful. I seldom find anything below 28mm to be interesting. 35mm is as much wide as I want to go while 85 is just sufficiently long. Having said that would not I prefer a 24-105mm f/2.8 over everything else? Of course I would, but sadly that is also not going to materialize.
Hopefully the 24-105mm f/4 from Sigma is at least as good as 24-105 f/4 of Canon.