Two points not yet mentioned.
Terrorist attack and insurance cover.
A dedicated, trained and suicidal terrorist unit taking over or disabling an en route control facility, with jamming equipment powerful enough to disable alternative control methods, even in a limited area, cannot be discounted now or until terrorism has ceased.
Right now the prospect of the aviation insurance market giving cover for hulls, passenger liability and third party liability are zero. A few maverick underwriters might be prepared to take a small share but none of the leading syndicates or companies would touch it. Imagine if two 300 seat aircraft either deliberately, as a result of terrorist action or a failure of equipment, were to collide overhead a crowded city centre and crash to the ground, the final bill would run to billions and the aviation insurance market would suspend all airline insurance, nothing would move.