The Airline (or portions thereof, would still be used if the Valley Line was fully operational. It would provide an interchange route to Cedar Hill without having to use AMTRAK trackage. Additionally, the TILCON quarry at the Gap is located on this line. At a minimum, trains from Cedar Hill would still operate between the quarry and the Hill.
There is only one other client on the Airline Branch between Cedar Hill and Middletown. That is Dyno, a blasting service company, located in Middlefield. This could be serviced from Middletown. Since they are located at, roughly, milepost 17 and the quarry restriction starts at MP 15, there would be a short "intermediate" area on the Airline that could be served from either direction.
The idea of the Valley Line is to lower costs by bypassing AMTRAK trackage between Hartford and New Haven. A lack of a customer base along either length of trackage is certainly a concern. The client base is in Middletown and Portland and could be served via the Valley. In a couple of years a pipeline is scheduled to replace the Poo Poo Choo Choo, which brings sewer sludge from Middeltown to Cromwell on a daily basis.
Any trains attempting to go from Hartford to Cedar Hill along the Valley Line would also have to have power on both ends, or at least additional power waiting in Middletown, because of the switch from the Valley to the Airline requires the ends leading end from Hartford to become the trailing end to Cedar Hill.
NIMBY'S are NIMBY'S til death due them part...or money talks louder. Of course, the ROW there is owned by the State of CT....who couldn't care less about railroads in any sense of the word. If the the State DOT had the guts, they would just tell the NIMBY'S to pound sand (and not over their rails).