The problem with the black boxes is that they won’t tell you much about the hijacking part 8 hours earlier — they will only tell you about the ditching/crashing part at the end of the joyride.
Real time tracking information will not stop a determined hijacker/pilot, but would at least eliminate a “missing” airliner. The route and final location will be known, preventing the lengthy anguish of not knowing what’s happened and the immense expense of just trying to find the plane.