Correct me if I'm wrong, but if memory serves me right, it's not that the 24-70 mk 1 is bad on absolute term, but more like quality gap/variation between each copy is quite big - and this is not acceptable for an L lens.
...24-70 mk I is a lot more inferior to mkii. The mk I is one of the weakest 24-70 in the market... especially in terms of sharpness..