I don't know about focus shift, but what supporters of this lens are terribly overlooking is that the 24-105 has a range and a price that leave room for compromise, whereas a 24-70 f/4 sold for 1600$ has to be epitome of optical perfection itself - and it's not.
I am sure many if not most people who own a 24-105 wouldn't feel the need to upgrade to a 24-70 IS. I am also sure that most people looking for a standard zoom for FF will still prefer a 24-105. I don't think that a comparison between the two lenses is the big question here.
The so-called "supporters" of the 24-70 are merely trying to reason that it doesn't make sense to "spank" a lens without basis. However, a justifiable basis would be if the lens consistently demonstrates the problem of focus shift (something that I recently learned about) or if Canon decides to replace the 24-105 with the 24-70 without lowering the price or fixing the issue (assuming it really is an issue across many samples).