I wouldn’t bother to carry around the 100-400mm to get the 135mm focal length. I’d just shoot at 105mm with my kit lens and crop a bit. I rarely need the whole 26MP of the 6D2. I realize that I would’t get the look of a wide open 135mm f/2 whatever I would do, but that is not something I have ever missed.
I don't use primes enough to justify truly expensive ones, but even the (relative) cheapies have a better f/number than the zooms. I do have the 85mm 1/4; 135mm would be a logical step up...again if there were a non-L, an L isn't worth it to me. (My 100mm macro is f/2.8 and I tend to use it for...macro. The relatively minor focal length difference between it and an 85 is outweighed by the 85's aperture.)