I recently picked up a 135mm for the RF645. They are out there but quite hard to find. I have read that Bronica only made 200 before discontinuing. I tend to use the 100mm more than the 135mm because it focuses closer. The 100mm focuses down to 1.2 meters, 135mm only down to 2 meters. For portraits this makes quite a difference. I figured I probably wouldn't see another 135mm for sale any time soon so I bought it. My camera is set up for with frame lines for the 100mm, I just imagine a smaller area for the 135mm in the 100mm frame lines.
I love the camera. I use it with 100mm lens more than any of the others. As far as discontinuing the 100mm lens, I would say that Bronica is just selling off the remaining stock of bodies and lenses. They probably ran out of 100mm lenses and still have some normals and wides to get rid of.