Optically the 24-105mm is better, especially at larger apertures. F4 throughout the Zoom range is much more useful than 3.5-5.6 especially at the long end. It quickly moves from 3.5-5.6 so you loose light and variable aperture lenses are just a pain. 24mm is much more of a big deal than 135mm on the longer end for me. Easier to have a 70-200mm which I always keep with me.
Also the 24-105mm is weather sealed and built like a brick in comparison. I never saw the value in the 28-135mm it was rubbish on crop because its like 38mm on the wide end and on FF the IQ isnt good enough. Value wise.. if your on a budget fair enough but its pointless adding a high quality body to a poor lens, better off buying a crop camera and a better lens.
As for upgrading it I would prefer Canon spent R&D time on an upgrade to the 24-105mm maybe to 24-135mm F4 L that will sell like hot cakes. If they made a 2.8 version of that...
that would be the perfect walk around. But they would never do it.. as it will make 3-4 lenses irrelevant, so il keep dreaming.
In fact im surprised they are still making the 28-135mm.