Here's an excerpt from 'How Airliners Fly', first published in 1997:
<<Although subsonic cruising speed has reached its natural limit, navigational efficiency is still advancing. Computers in Air Traffic Control centres and in airliners themselves allow more productive use of airspace as the density of traffic in the sky steadily grows. One likely development is greater direct control of flights from the ground, rather than by spoken radio communication between pilots and controllers as at present, with its attendant potential for misunderstandings, especially when communications are in English between participants for whom this language is not the mother tongue.
It is unlikely that complete control from the ground will ever be achieved, however, because there will always be occasions when problems can only be resolved with the judgement of a human mind in the flight deck, such as the necessity to deviate from the planned flight path in order to avoid hazardous weather or for technical reasons or to reconfigure aircraft technical systems after faults occur. Perhaps a system will evolve where a ground controller sends instructions directly to an aircraft's autopilot but the instruction will not be executed until the human pilots permit it.
Would one pilot alone on the flight deck be enough if Air Traffic Control were 'flying' the aircraft? It is likely that one person could handle the workload during normal operations. But suppose the human pilot considers it necessary to intervene. Who will be there to assist him or her, and more importantly, to confirm or query his or her judgement and to monitor his or her actions? Perhaps as long as there are airliners in the sky there will always be a case for at least two humans in the flight deck.
Again, will our future pilots be able to fly their aircraft without the assistance of autopilots and computers when necessary if they never get the chance to practise these skills during normal operation? A related factor is that a pilot whose job is merely to watch the aircraft fly itself is unlikely to be as well motivated as one who can get his or her hands on the controls now and then. Designers of future aircraft and airline managers must address the issue of how much and under what conditions pilots should be allowed, or indeed encouraged, to fly manually and without guidance systems. It is likely that compared to a mere aircraft monitor, a skilled, motivated pilot will always make a greater overall contribution to flight safety.>>