Injection moulding is a volume game, and I suspect that anyone with enough money to invest in tooling would get a better return by commissioning RTR coaches from China than trying to produce the same prototype as kits.
It's interesting that Slaters moved out of 4mm kits, some time ago, and licensed their tooling to Coopercraft, it suggests to me that, even then, the volumes of 4mm kits they were selling did not justify commissioning new tooling to extend their ranges.