Another little point in addition to those already made: For all of the systems installed, there is a "get out of jail" procedure involved, should it quit or start to work without being called to duty.
What about Your idea in such a situation? What if the retro rockets fire on a performance-limited takeoff and eat up the required performance? What if they fire after an engine malfunction that is otherwise under control but does not leave the aircraft with much remaining performance? The DH8-300 on a single engine is not exactly a performance wonder without this already. What if the parachute opens without being commanded while the aircraft is over nasty terrain but with landable, flat fields within gliding range? What if the parachute or the rockets fire in an approach at 1.3Vs, even more so when single engine? Also, what happens if the braking rockets fire, but the parachute fails to open?
You see where I am getting at. The desirability of such a system quickly becomes a statistical exercise. How often will such a system be of benefit to the aircraft, and how often will it turn an otherwise controllable and often trained situation into a non-manageable mess ending up in a total loss? If benefit outweighs the risks, it is worth a try. But if it ends up increasing the risks to the aircraft, it is highly undesirable to have such a thing on board.
I did not do the maths behind this. But seeing that a single engine failure on a twin engined airliner is very much within its performance envelope, that there were very few accidents if any where such a system would have saved the day, and such a system weighs more than a few grams and requires massive structural adaptations, new cables, pyrotechnics, controls, a maintenance program and so on, I would not think this to be a worthy addition to an aircraft.