Protocols can certainly be developed that would minimize the chances of a ground crew hijacking an aircraft remotely. For instance, make sure at least two completely random separate control centers agree on the takeover simultaneously.
Nothing in our lives is certain, but we base our actions on realities, not absolute certainties. The technology exists to greatly reduce these kinds of tragedies. I say we use it to its greatest potential without losing focus on the human element as well.
Such events are exceedingly rare. There are about a million people airborne _right_now_, and at any given time. You’re talking about _one_ flight going down this way once per decade, working out to roughly a 1 in 22,000,000 chance of a passenger dying from suicidal airbus pilot. At some point, the returns for implementing anti-terrorism (indistinguishable from human subjugation) technology are so vanishingly small and so outrageously expensive that, truly, it’s better to spend the money somewhere else.
Way more people will die today from auto accidents alone than died on the Germanwings crash. Yet...you’re not prioritizing accordingly. Yes, the technology exists to mitigate such incidents - I say we apply the money & tech somewhere else where far more lives will be saved & prolonged.
Remember: a non-locking door and an armed passenger could have saved that flight. That’s a whole lot easier & cheaper than all the anti-terrorist protocols & tech we’ve wasted since the days cockpits didn’t have doors and passengers could CCW on board.