It takes four positive wires and a negative to control dynamics in the current AAR standard arrangement. The brake warning light, DB setup and DB turn on aren't a problem because they are on/off circuits. The problem is the DB control wire which varies in voltage to vary the amount of excitation and resulting dynamic braking effort because of the resistance in the length of the MU wiring.

Of course, this and the ammeter feedback could be controlled by other means as a digital feedback system does exist, but as Dr. Z said it is a matter of money. The new digital braking system has been promoted to include MU and DPU capability as part of its proposed implementation.

Unless the cab car is going to lead a train down a long grade with a substantial speed restriction, there really isn't an absolute demand to control the dynamics separately. Typically the cab cars used on regional or commuter trains are leading trains that need to run at or near full throttle to maintain the scheduled speed, or they are grabbing all the brakes they can to get slowed down at the last minute prior to a speed restriction or station stop. The blended braking arrangement provides an easier and more stable control of the train's brakes in these instances (assuming it is properly engineered and working correctly). I don't think many modern operations have used extended range so you would have to deal with DB brake fade to nothing below 25 mph.