an f/1.4 wide-angle lens is NOT about speed. It is about the chance to get at least some halfway decent bokeh at wide-angle focal lengths. An f/1.4 lens therefore can never be substituted by a slower lens with IS.
The real issue however is not IS or not IS, it is the fact, that all of Canon's current 1.4 lenses deliver SUB-PAR IQ wide open. NB matter whether they have a red ring or not. 24 L II, 35 L I, 50 non L. And the 50/1.2 L is a sub-par piece of cr*p as well by todays standards. A Sigma 50/1.4 runs circles around it.
As far as 35mm fixed focals are concerned, currently the only 35/1.4 in the entire market which is fully usable at f/1.4 is the Sigma 35/1.4. At a street price wich is lower than the totally useless Canon 24 /2.8 IS and 28/2.8 IS and it beats the Zeiss Distagon 35.
And btw, I come from an entirely different usage situation:
* when I shoot handheld in low light, I ALWAYS shoot moving targets ... that is ... PEOPLE in motion ... 1/60s needed, nothing less. IS useless.
* when I shoot static targets in low light, I ALWAYS use a tripod. IS useless.