Adding IS isn't all positives. It adds significant size, weight, complexity, and the biggest issue for most; cost. Best case scenario, IS will only add $500-600 to the cost. That's still a fairly significant sum. I'm sure many people that don't want or need IS would opt for the non-IS version.
There are many lenses with IS that cost less than $500 - so the IS system itself cannot cost that much. They may charge that much more because they *can*, but not because they have to. When comparing the 70-200 f2.8 IS and non-IS, we see about 10% increase in weight and a slight increase in size. On the f4 versions, the weight increase is about the same, but the size increase is negligible (+1mm diameter). The optics are more complicated in the IS versions, but the image quality isn't worse - especially not if you consider the new mkII IS version.
My point is merely that Canon could *choose* to release an IS version of the 24-70mm f2.8 without sacrificing performance, weight or the price point if they wanted to. They will almost certainly release the new lens at a significant price premium, at the very least until supplies are high, but that will likely be a business decision rather than a engineering necessity.
I'm sure I'm not the first to say it, but with Canon's position/reputation in the market as the video leader but low-light loser, putting IS on the standard zoom ought to be a priority for Canon. First to solidify their lead in video performance and secondly to counter their reputation as trailing in low-light performance.
TL;DR? I think you're overestimating the problems with IS, and I would be very surprised if a new 24-70 did not include it.