Sigma's and Canon's recent efforts really just show how hard it is to make a 24-105mm lens and, all things considered, how incredibly good the original Canon 24-105 is.
A zoom going from UWA to medium telephoto is much more optically challenging than a pure telephoto, such a a 70-200, 100-400 or 150-600. The 4.4x zoom range is also much greater than on other wide-angle zooms. Not only that, but the range of focal lengths involved requires both a retrofocus design at the short end, changing to a telephoto design at the long end. You don't need the same compromise to make a 24-70 or a 70-200.
This is probably why we haven't seen a 24-105 f/2.8 yet. You could build it, but it would be big and heavy. Since it doesn't replace the 70-200 in the kit, most people would also have to carry that around. And anyone willing to carry that much weight around is probably after a bit better image quality than a 24-105 can deliver.
I doubt we'll see too much improvement in 24-105mm designs at this price point, until electroactive polymers replace glass elements and allow for focal length change without having to reposition elements.