As has been said here many, many times before, notwithstanding passengers being dead against it, it is security that will stop it from happening. A bunch of dedicated suicidal terrorist could take over a ground control unit, possibly but not necessarily, in a country where security may not be as tight as in others or they may just get them selves their own ground control unit that can block the signals from the genuine ground unit, either way mayhem and blood shed will ensue and no further passengers will board a pilotless aircraft. It simply won't happen.