Don't have the Zeiss 35mm f1.4 neither the new canon 16-35 F/4, but I was not happy with the 16-35 f2.8, and I am keeping my canon 35mm f1.4, because the AF is a big plus for me at this focal range.
So, from the 3 lenses you have, I would only sell the 35mm - no way I will sell either my 15 and 135 copies. Maybe in 10 years or longer, canon will make a better 15mm (or 14) and 135mm but, I bet the Z's will still focus and work fine then... and there is a good chance the canon usm motors burned out already
If you not using either one so much, then maybe it's time to try new a new focal length, or if indeed you switch often between the 15 and 35, a zoom as good as others claim - the 16-35 f4 - will be more usefull for you.
In sum, check back on which focals you are using the most in your shots; that will tell you the need to keep the lens or not
- a lot of valid points already given to you, and if you don't see the IQ difference, then by all means sell them all