What was 'wrong' with the 300mm f/2.8L IS or the 500mm f/4L IS? Not a helluva lot. But Canon would rather sell you the MkII versions costing 50% more. If nothing else, the 135L lacks weather sealing. But consider...Canon developed a zoom lens (70-200 II) that pretty much equals that excellent prime for IQ. A prime should be better, and with modern design the 135L could be better. A 135L II would not surprise me at all.
I have a fair collection of lenses (including 9 "L" lenses), and while I am very happy with some of them, there are a few where I wouldn't mind getting an improved version. Now, in my case, the 135mm lens is one of those I am very happy with - I do not consider the lens to be limiting me in any way, and even if there was an upgrade, I probably would not switch...and I don't think I'm alone in having that opinion.
Sure, some people might be tempted, if Canon introduced a 135mm II version with IS, but otherwise...well, I just don't see this making any sense. Old or not, it is just a top-notch lens.