I see these two lenses to be complementary to one another. For me, neither can replace the other.
I have the new 16-35mm IS....love it...and I also have the (better companion IMHO because is has similar sharpness and contrast) Canon 24-70mm f/2.8 II.
Rounding it all off with a 70-200mm f/2.8L IS II.
Yes..the first two lenses overlap...but I would generally grab one for much different shooting than the other...
Two different applications in my world.