As an NYC Operating Management Trainee, I got involved in several projects like this and I know whereof you speak. Even after you plugged in the “knowns” and the estimated allocations, there was always that nagging question of whether you would have the traffic at all if the branch were abandoned. In other words, would you have continued to receive the line-haul if the branch line to the consignee were abandoned. My thought was always “No” but I was usually overridden.
The same basic question always came up concerning local vs through passenger service. Without the locals feeding the through trains at connecting stations, how much traffic will the through train handle? It always seemed silly to think that customers would be willing to use their private automobiles or other local conveyance to get to the (former) connecting station but not to just continue to their ultimate destination without the through train.