This doesn't contradict my vague scenario, but I wrote it after having read the posted article (about the clever ham radio hack) but before reading Langweiche's article which was mentioned in another comment.
There are a few theories floating around about how this happened that suggest a possible narrative that explains this. Essentially: by the time the in-flight maps would have shown anything weird, it was too late, and shortly after that, everyone outside the cockpit would have died. My paraphrased version:
Right before Zaharie made his first (unscheduled) turn, he could have gotten the first officer to leave the cockpit, probably by telling him that one of the cabin crew needed to see him in person about something (the first officer was a young rising star in the airline, and likely would have followed his superior's instructions without question). He'd then lock the cockpit door, put on an oxygen mask, and depressurized the cabin, maybe even cutting power to the cabin lights to make things more confusing.
Before anyone could react, he'd start the first turn. Based on simulations done, it was a pretty violent turn, pushing the plane to its limits; the first officer and many/all of the cabin crew would have been on their feet, and would have been knocked around to the point that they may not have been able to get oxygen masks on in time to prevent unconsciousness. Even if any of them, including the first officer, could get a mask on, there's not much they could do. They would assume there was an emergency, but the first officer would be unable to get to the cockpit (locked, plus the need to be tethered to an oxygen mask's supply), and the cabin crew would generally not try to get into the cockpit in that situation, but would attempt to secure the cabin and do their best to manage the passengers.
Most passengers would probably be able to get their masks on (if they were already belted into their seats), but the oxygen in those masks runs out in some surprisingly-small number of minutes: in the event of depressurization the masks are a stopgap measure to keep everyone conscious/alive while the pilot descends to an altitude where the masks aren't necessary, but in this case the pilot intentionally did not do that. Most, if not all, passengers would follow the steps drilled into them by the safety video: put their oxygen masks on, help anyone next to them, and stay belted into their seats. They'd assume that there was a legitimate issue with the plane, not that the pilot was trying to kill them.
Even if anyone noticed the in-flight maps during that time, they wouldn't have much time to puzzle over it, and would likely assume that the weird course was a result of whatever emergency was going on in the cockpit.
Perhaps there should be a device capable of sending a distress signal accessible from the cabin by flight-attendants?
A similar solution could also be applicable to other means of transport. E.g. A similar solution would have been extremely useful for the indonesian submarine lost weeks ago.
Which is one of the elements of MH370 that suggests a deliberate act: it did go almost entirely dark, but for the hourly satellite pings. If the flight recorders are ever found then it may be learned if this was so, and whether it was malfeasance or part of a lost fight against malfunctions/fire.
I think "crew" is the key part here, right? I understand a system like this would be designed in such a way a single crew member could not make a plane go dark -- or make it stay dark after disabling the safety systems. For example, a rogue pilot can manage to turn the beacons off, but any other from the cabin crew can just turn it on again while the pilot is in the cockpit, etc.
I still need to find out which changes this even brought to the entire airline ecosystem.
Giving a flight attendant a backdoor code would make them more susceptible to a hostage situation.. It's one of those damned if you do, damned if you don't situations I guess. There is no perfect answer in the situation where the most trusted person is compromised without compromising security in all the other scenarios.
Jumping off a bridge seems quite difficult in the moment. Whereas it may be much easier to simply do what you always do, being distracted a bit by the need to execute details (that may feel emotionally innocuous) until you gradually slip across a point of no return and the decision is out of your hands. You could even work on controlling the plane all the way to the water, just as you were trained to until, of course, there was nothing further you could do.
I do imagine that some passengers and crew must have noticed something weird about the in flight maps. Almost the entire flight path is out of phone range though so unless there was satellite internet there would have been nothing any crew could have done. Cockpit doors are armored these days.