I'm not convinced that they couldn't add it and improve IQ at the same time, given improvements in software used for lens design, manufacturing tolerances, etc . I don't think you can really jump to any conclusions in that area based on the inclusion or exclusion of IS in some of the existing lenses. Look at the latest Canon 18-55mm kit lens as one example, (it's much better optically than the older one without IS, but I doubt the addition of IS is the reason).
Canon probably knows whether or not they can achieve optical quality as high with it (or not), as I can imagine they've looked at a number of designs for any future upgrade or remake (if it's even a high priority with them). If it wouldn't degrade optical quality, I'd probably add it (giving them a feature that the Nikkor 24-70mm f/2.8 doesn't have for photographers that may want to use it in some conditions). Given that this a premium grade lens, I'd want an advantage in every area I could find one in.