I think at this point an intrinsic part of the game is managing productions based on those specific production lengths. We have to decide when to choose 3 hrs and when to choose 9 hrs. If there were suddenly loads more time options then we would lose that planning aspect. In a way it's a similar argument to the one when people were asking to be able to put buildings away in order to reorganise their cities... Still, that one eventually did get implemented so what do I know.