I think the reduction would be significantly less than 50%.
There are “sunk costs” that likely neither grow nor shrink at the same rate as the population.
Think of it like a married couple. They have a lower utility bill with a single house than if they had one house each. They may also share one vehicle. If one of them was gone, their annual expenses would not necessarily be cut by 50%. Same would likely be said for carbon footprints.
If we had less people, we would necessarily have less refineries? Perhaps even worse, if we had less people, would we even be compelled to continue the efficiencies we currently use or would it be an excuse to pollute more? Would less people lead to more affluency? Would that affluency lead to greater energy usage (it usually does)?