A cabin crew member in the toilet will not prevent a determined mental suicide event from happening by deliberate pilot action. Case in point, suicide pilot and cabin crew member in cockpit. Suicide pilot decides to shut down engines and turn flight control computers off as well as carrying out a rapid decompression of the aircraft. Let's see if any cabin crew member would be able to turn that lot back on while the suicide pilot has jumped out of their seat to put the dead bolt on and started attacking the cabin crew member.
The only true way to prevent this ever happening again is for 2 pilots to be present in the cockpit at all times. Financially this will never happen as many carriers think that the cost does not outweigh the risks.
How will airlines prevent the issue of a suicide pilot nose diving the aircraft a few hundred feet up during climb out off the end of the runway. What about a landing incident a few hundred feet above a highly populated City. The prevention here is not in the cockpit, it must be done "somehow" in the doctors clinic.
I am not sure how the future will play out but some test for level of mental state or craziness is no doubt going to be implemented into medicals in the future. Guys undergoing divorces, people with money debt issues or whatever might be moved up into the higher risk braket. This is an cruise issue that can "only" be resolved by having 2 pilots in the cockpit period, but will never happen due to the accountants, they are the root cause of this and many other safety issues in airlines.
J