In what modern situation would a whole plane parachute actually help save lives? The vast majority of the (extremely rare) commercial aircraft disasters occur during takeoff or landing, when a parachute would provide little utility due to the distance the aircraft is from the ground. Other notable recent disasters had either instantaneous destruction of the aircraft (Metrojet 9268, Malaysian 17) or were caused by pilot error or murder/suicide (Germanwings 9525, Colgan 2407).

Perhaps the only incident I can think of where a parachute may have helped was US Airways 1549, where a bird strike caused loss of power to both engines. In that case, however, sufficient safety controls existed to enable landing on the Hudson river, and the aircraft functioned as designed (engines broke off when hitting water, the aircraft floated long enough to ensure safe evacuation of all passengers, life rafts deployed, etc). I would argue a parachute would probably have resulted in loss of life as a giant A320 parachute falling uncontrolled on to New York City would probably kill people crashing in to a building.

Rather than focus on superfluous, impractical safety measures, commercial aircraft designers have instead spent time on things that actually save lives, such as Traffic Avoidance and Ground Proximity warning systems.

Note that parachutes do exist for smaller planes, where the risks and benefits are substantially different. A good example is the Cirrus SR-22. It seems that most cases of deploying the parachute on the Cirrus is due to pilots running out of fuel, a failure which is extremely improbable in commercial aviation.