Just some thoughts:
To me the new front fascia upper grille seems to have less area which I suspect would offer less cooling on a car where heat is already somewhat of an issue. Assuming that is true they are not going to hurt the performance by updating the cosmetics. It is also possible the downforce levels would be changed in a negative way as the ZL1 hood is different from the new vented hood. So I don't think it is being lazy, just knowing they perfected the design already and don't want to mess with it. Clearly they took cues from the ZL1 for the new fascia.