I'm a little puzzled at this one. It's not that I doubt that Sigma can improve on the optical quality of the 24-105L, but that lens is praised for being competent and, as has already been pointed out, it represents a strong value at this stage in its development. It could be sharper, it could have less distortion, it could have less vignetting, but it's not terrible in any of those areas either. It also has a very robust build quality and weather sealing (both of which I have had occasion to test in the two copies I've owned in the past). Sigma could probably improve in all of those areas (save build quality, perhaps), but I struggle to see where they could turn any of those things into a killer reason for the myriad 24-105L owners to change lenses.
Sigma has been filling a lot of unfilled niches recently, but this is anything but that. Curious.