No IS + F/1.8 = No replacement for 135L.
So the IQ of a lens would be great if it had IS and 1.8? I am sure that someone like you knows that there is much more to a lens: Corner sharpness, build quality, hard stops, Bokah, fall off, sharpness at f2 etc etc. Besides 1.8 or 2 does not make much difference in real life photography.
Not saying that. Just saying you get to a point where comparing razors to razors like the 135L and the zeiss that its pretty much irrelevant. For me, The issues I run into is the lack of IS and I could use a bit more speed.
Another sharp 135mm f/2 is nice but I see IS and 1.8 to be much more useful.